There are numerous HYIPs out there, and a majority of them are scams, so how do you find which one are scams and which ones are legitimate. Below is a list of things to do that will eliminate a majority of scams.
Go to HYIP Forums
Do a 'whois' search
Check if the HYIP is hosted by a free web hosting site
When was it registered
See what the HYIPs contact details are like
Test the contact details
Read over the website and see if it has any thing that looks suspicious or bad engrish
Know what the HYIP does to make its money
Returns that are too good to be true usually are
Watch out for HYIPs with very high referral commission
How long does the HYIP hold your principal for?
Always do a test spend and withdrawal

1. Go to HYIP forums
The first thing to do is go to a HYIP forum and look up the HYIP that you are interested in. Read everything that the users have to say, but usually take what the veterans (500+ posts) have to say over someone who has very few posts.

2. Do a ‘whois’ search
To do a whois search go to somewhere like www.whois.net or whois.webhosting.info and type in the URL of the HYIP. These services display a lot of information such as who registered the domain their contact details, when it was registered and even where they are hosting the site.

a. Check if the HYIP is hosted by a free web hosting site
Using the hosting information from the ‘whois’ search you can determine if the HYIP is on a free web hosting service. If the HYIP is hosted by a free service you can be pretty sure that it is a scam.

b. When it was registered
If a HYIP claims to be 5 years old but the ‘whois’ search says that the domain is only a couple of months old, you can be pretty sure that it is a scam.

3. See what the HYIPs contact details are like
Go to the HYIP site and check out their contact details, the more ways to contact them the better, eg Email, Phone, Forum, Address, etc

4. Test the contact details
Send an email to the HYIP admin asking a legitimate question. If they do not respond then it may not be worth investing with them. If they do reply but it takes them many days/week to do so, again it may not be worth investing with them.

5. Read over the website and see if it has any thing that looks suspicious
Go to the HYIP web site, click on all the links and see if that actually take you to where they are supposed to. If the designer has done a poor effort in designing the website then they will probably do a poor job with your money too. Also keep an eye out on what they are writing too; after a while your may notice that some HYIPs have exactly the same words (word for word) on them (in the FAQ, news, etc). It is clear that they have copied it from another HYIP site and that they are probably scammers.

6. Know what the HYIP does to make its money
Find out how the HYIP makes its money. If the HYIP claims to make its money through “a secret banking loan” or something similar they are a scam. If the way they invest money also sounds too risky/dodgy then don’t invest in them.

7. Returns that are too good to be true usually are
The old saying ‘if it is too good to be true’, you will find that it probably is. If a HYIP promises hourly payouts it will most likely be gone long before you can get any Return On Investment (ROI). Further more if a HYIP promises ROI of you initial capital in less than a week it is also likely to disappear soon and become a scam.

8. Watch out for HYIPs with high referral commission
If a HYIP offers a referral commission above 15% it implies that it needs more members to survive. This indicates that the HYIP is most likely a Ponzi scam and should be avoided.

9. How long does the HYIP hold your principal for?
Check out how long a HYIP holds your principal for. A majority of HYIPs go under well within a year and it also ties your money up in a single program.

10. Always do a test spend and withdrawal
Always do a test spend to see how the HYIP performs. If a HYIP does not pay you when you make a test withdrawal then feel lucky that you have only lost a small amount of money. Payouts should not take more than 48 hours after a request, if it does this is a sign of concern if they do not have a good reason.

Article from www.hyipcastle.com

From what you are probably aware of; there are a lot of HYIP opportunities that can be considered good and bad. What you are probably wondering or not aware of, are ways to evaluate them before you go out and lose a lot of money on them. Everyone is probably aware that any type of hi yield investing poses risks, and these risks are always there.
Any type of HYIP will contain characteristics that will slant them in one direction or another. It is by evaluating those characteristics that are able to sort them out. Keep in mind that the HYIP proponents are keen to confuse and issue, and deflect attention, but you can dig through most of that if you know what you're looking for. Below, you find some examples that you will assist you in your decisions and research.
- Games of chance v investments: There are hundreds of games sites on the internet, many of which cost money to enjoy. While there is certainly nothing wrong with that, they obviously have no place in our consideration of hi yield type investments. They should be identified, and thrown out whenever they seep in. - Recyclers' v generators: A lot of HYIP out there like to invest in the same thing. While this may be ok, there are a lot of traps; especially if this is hidden, or denied, and the expected results are overstated. - Short term v long term: All HYIP's that offer excessively fast returns are almost always doomed. Because of this, you should beware of the excessively fast money merchants. - Real v virtual money: A lot of the HYIP's pay your returns into their own accounts, and tell you that the money is there, often with restrictions on how you can get access to it. This method is very widespread, and is usually ok, but still needs to be tested before you invest a lot of money into it. - Fixed interest v variable return: As you probably know already, fixed interest looks nice. Although it's usually not consistent with actual earning rates, it still raises a conflict and sometimes complete failure. A lot of the best HYIP's deal with the problem simply by offering low quotations of minimum return, although its better to be dealing with someone who meets those standards, than someone who repeatedly defaults on more optimistic targets.
With any type of HYIP, you should always consider them to be bad until you have proof that states otherwise. Don't get the wrong idea; there are a lot of these out there that can be very beneficial. Before you invest your money, you should make sure you know what a good and bad HYIP really is.