You read the whitepaper, you see the team behind the project. If it's something that has been done before multiple times and it didn't give any success (say a debit card for crypto) or you have the least doubt that they won't manage to deliver, simply don't invest.
I agree to an extent, but not completely. The WP says a lot about the project, the team too, development, planning and road map also. But just because a thing's been tried before and failed doesn't mean the next person doing it is a scam. And for the debit Crypto card you mentioned, I think this is coming soon than later. MFChain, a cryptopayments platform is coming out with something like this and I very certain, they're the farthest thing from a scam you can think of.