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.