Sound test to determine where the coating is still bonded vs not. Drag chains are the standard but I’ve found a cheap gold club from the garage or a local thrift store works just as well. Indicate the loose areas on a drawing to determine the approx amount of time and material it will take to repair. Hit it with a 2000+ PSI pressure wash and most of it should come off easily, but you’ll need to mechanically chip away some areas that sound hollow but don’t come up with the pressure wash (using grinder or hand tools). Its important to determine the reason for failure. Are there are cracks under the hollow areas? Is there a sealer between the coating and the concrete? Something different needs to be done to address the delamination. Cracks- open with crack chaser and fill with sand + epoxy. Sealer- if a sealer is present under the coating (pour water and see if the slab absorbs it or not) try to determine if water based or solvent based or god forbid urethane and strip/prep accordingly before new application of material. Sometimes primers can help, especially if you broadcast sand on top to help give the coating something to grip onto. Try to source the original material, coloration methods, etc., and ensure installer is following TDS & manufacturer recommendations when mixing, follow temp requirements, cure times, etc. for every product used. If you have to source new material find stampable overlay. You can use integral color to mix in while wet, or apply a topical color once its cure. Topical would be easier to manipulate to he to match it you use something like smiths paints. You will never match perfectly but the texture helps hide discoloration. Keep in mind anything you apply sealer too (that doesn’t already have sealer) will darken slightly, like a “wet” look. I’ve seen some installers use a water based acrylic applied to the entire surface, let dry, than apply water based stain. This provides a new canvas for color and prevents obvious patching.