Several things to look for. Overall color, be sure it is fairly uniform between each branch and their corallites. No white patches, black lines or blotchs. Thinkness of the branches based on species. A well maintained chem will mean a much thicker healthier coral while a chem outside NSW
(higher) will be thin, reedy and have brittle branchs. It can also cause uneven thicknesses along the branchs. If buying frags
on plugs, be sure they have some amount of growth covering the plug itself. That will ensure it is not a recent frag
and still a little "iffy" on survival.
Also look for signs of parasitic activity in the coral holding tank itself. If the "general" health of the tank is lacking it's best to steer clear unless you can easily and confidantly dip the coral. A main one to look for with Acropora's is red bugs although not usually a concern with other SPS
species and tiny little nudibrancs
on monti caps. Usually found on the underside of the coral.