They should be on the latest version of the stable Steemd.
They should not be disabled.
They should have minimal missed blocks.
They should be publishing a price feed.
Those are really the only major qualifiers to look for without historical data. In the future, I am going to add charting and past information so a user can make a better informed decision.
PS: Disabling a witness is not bad, if it remains disabled for a long time that is bad.