How can we recognize new coins and exchange, what legal paper have to be on them website on that we can assure coin and exchange is trustable?
There is no such as legal documents for a coin. Perhaps some Exchanges have licence documents which is not much useful for traders. More important is how trustworthy an exchange. Only you may read on whitepaper about a coin, their future development plan and what is concept. As we can see most of project last year's are not successful even after listed on exchange. There were some sudden pump and dump again. For general traders like us, better to avoid dealing with new coin after listed on exchange. Just wait few months and look on the movement. Likely you would get more better chance to get profit from it. If not, then just skip, at least you had not lost anything.
ICO based and fueled exchanges that use their own coin for lower fees or profits always seem to be abundant and then disappear. I'm more willing to trade small amounts on smaller exchanges especially when dealing with altcoins but if I'm dealing with fiat in any form I usually a super legitimate exchange, and I suggest others do the same even if this means you need to report tax, that's just a bit of life.