Well, the truth is the cryptocurrency market is too broad for any one exchange to cover all coins and tokens. Most people would think that Coinbase and Binance have covered the entire market since they've been around for the longest. However, there are so many exchanges trading across the board.

Depending on which altcoin you want to invest in, most exchanges can be found on sites like CoinGecko or Coinmarketcap. The easiest way to verify whether the exchange is good or not is according to the feedback of other users. Check the ANN thread of the exchange here on the forum and search the Scam Accusations board to see if there are any complaints. You're going to have to verify things on your own, because it's not always a smart idea to blindly trust others, so you need to do your own research.