To avoid scam DEX, just look at those exchanges that have been operating for a long time and have a good reputation. For example, forkdelta.app is suitable for ERC20 tokens, stellarport.io is suitable for tokens based on Stellar, you can use tronwatch.market for tokens based on TRON.
Or you could use Blocknet's DEX, which has been around the longest. Early next year it's gonna be way more convenient to use due to the release of a lightwallet with the DEX integrated into it.