It will be difficult to get a solid proof without lawyers involved to check everything with the third parties. Typically you just want to have a written statem

It will be difficult to get a solid proof without lawyers involved to check everything with the third parties. Typically you just want to have a written statement from your founder and if at some point the company is worth enough to justify a check, you can hire lawyers to go back and check. If your founders lied in a written statement they will be liable for even more. In general if your founder lies to you and you have proof and the company goes big he is in trouble. Your lawyers will need to see all paperwork you signed in such case, watch out for sketchy terms you might be asked to sign later on. It is possible that crappy founders will try to erase all prior liability at some time when the company is about to make it big. In fact the company lawyers always go back and check everything the founders signed before a big deal to avoid any unforeseen liability. You have to judge if your founders act in good faith or they are taking every opportunity to put you at disadvantage. If you don't trust the founders or the investors, don't do business with them.