Whitepaper can never be the only thing that can determine credibility of an ICO. However, NOTHING can be the only thing that determines the credibility of an ICO. You need to take a look at the whitepaper of course, it is one of the most important parts.

You should take a look at the team because the team also plays an important role, if you see a name that has been someone that really has been around for years in crypto space and has been a trustworthy individual and plays a key role in that team that is good too. You need to check what problem that coin tries to solve, you need to check if community is excited about the solution. You need to basically check bunch of different stuff to decide if an ICO is credible or not. Not just one.