How, for example, do you check the reputation of an Exchange? Are you just asking on btk or use some other sources?
Google the name of the exchange + "scam". When people have problems they post about it on reddit or in forums like this - and it will come up in the search results with "name-of-exchange scam".
Read all those posts to see what the problem is. Sometimes it's obvious that it's user error and they're just blaming the exchange because they don't want to admit they cocked up. But other times, they highlight serious problems with teh exchange. Look in particular for difficulties in withdrawing funds.
There will always be people with problems, doesn't matter the exchange. 50% of the people investing in crypto is a bit dumb.
The most important thing to look for is that you can withdraw your funds when you want, have 2FA, low exchange fees and liquidity. Binance is one of the best exchanges right now, I also like Kucoin, but there are some DEX trying to highlight.