How do you trust an investor not to steal your idea?

The first part of this is the legal side - the NDA - NEVER NEVER disclose any business idea to ANYONE you are thinking about going into business with without signed NDA's in place.

Second part is harder, business takes trust and if you are looking for an investor you need to be comfortable with your investor. DO NOT just jump if they wave a wad of cash under your nose. I did that a few years ago and ended up with a shark and lost one of my businesses. Only afterwards did I find out that everyone else knew he was a shark except me!

Do your home work and your due diligence on them. Research them, ask for previous business contacts / investments you can talk to, take the time to get to know them before you sign anything.

An investor is like a marriage and you both need to be comfortable if you are to be successful.

Good luck!

Make them sign and date a contract clearly stating/explaining your original idea and your property right to it.

you never can... there is always a way to find a generic version? for the sake of reality dont trust that investor and dont tell them about the idea and test it elsewhere

One of the above answers alluded to it.... you need to get an NDA (non-disclosure agreement, a.k.a. confidentiality agreement). Actually, it's best to get a non-disclosure AND non-use agreement -- requires investor to promise not to share your idea with others and also not to use it himself/herself.

Good luck.