• It shouldn’t look like it is struggling to swim (unless it is characteristic of the species e.g. Hawkfish or Wrasse). 
• It should not have trouble maintaining its position in the water.
• It should have no sores, pimples, dents or bumps.
• Its fins shouldn’t be folded or split.
• Its eyes should be clear and not uncharacteristically protruding.
• It should be breathing regularly.
• Its gills should be closed and not inflamed.
• Scales should be flat and intact.
• Its anus must not be red or inflamed.
• There should be no blood on the body or fins.
• There shouldn’t have white dots, or powdery or furry growths on the body or fins.
• It shouldn’t have a bloated stomach.  
• It is advisable to compare it to the other fish of its kind that you have seen or are in the shop.

Good luck & enjoy, 

Paul Talbot