I was 16 when i found out about these hyip sites (already 5 years ago). I believed what was written on the site. I invested my pennies i got from working hard at GPT sites.
I lost it all. I invested that time in about 5-7 hyips. I losted it all. So a golden tip i can give is: be CRITICAL.
600% in one day haha... ok Its not kinda reliable...

I got some other golden tips:

1. Watch the design
There has been a hyip scrip spread around.. (Legal/illegal version of it). Everyone, yes everyone!, can make a hyip. Write what he wants and scam people if he wants. The script that been used was from goldcoders. When u see that script,
never invest in it. http://www.goldcoders.com/demo4/ is an example how the hyip can look alike. goldcoders gives also other templates asswel. So be very careful when u invest in such site. To be exact 100% of all the hyips with that script are scam!

I gonna give some examples of other with a good design.

http://stableinterest.com/

http://www.geniusfunds.com/

...

2. Watch for a address / telephone, pictures of buildings,...
If you find a address, try to type it in on google maps. Does the building look like an office? You know people in that region,...

3. Talk to the admin
Ask the admin how he earns his money exactly!

4. Ask the admin more questions, ask him questions about things he need to know if he claims to be an investor...

5. Make the ratio profit makes sense?

6. Google the hyip... maybe google tells you something about if he has paid people out

7. Check on monitors (like guarddogmonitor.com) if the hyip paid people out

8. Check the support
your never able to get live support , it may be a sign. Or try contact them , see if you get a reply.

9. Check talkgold, check other forums about what the people say of the blog

10. Come and talk with us
(: You can find our yahoo at the support page, We can tell you in 1 minute if the hyip is a scam or not. We can advise you with our experience in this crazy world.

Invest what you can loose is my final golden rule and be a critical! nothing is to good for you!