look at every single part of its fins. That's the first thing for me. Next see if they are skinny or not and if they have cloudy eye (very unlikely).
aslo see if they still have both barbels and if there are any signs of body damage such as scrapes.
Bichir in my tank are finicky eaters. All bichir will eat eventually. Maybe not in front of you. I would say make sure scales look good. No frayed fins and make sure eyes are clear and not foggy.
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