Basically you are looking for obvious tissue loss. So here what I do.
Look for polyp extension. Sometimes the coral will not be extending it's polyps and everything is fine, but most times it is a sign that something is wrong. So expect to see some polyp extension with the exception of the coral just being added to the tank or recently handled.
Tissue loss. Look at the branches, especially in between and near the base where all the branches meet. There should be no tissue loss at all. Also pay careful attention to the base, there should be no tissue loss there either. Before purchasing the coral have the LFS employee take the coral and bring it close to the glass and turn it all the way around so you can inspect it from all sides for tissue loss.
Bleaching, the coral should not be bleached. But many times LFS coral are. If you see a super bright colored acro look at it closely, many times it is bleached. Now, I'm not going to say that I have never bought bleached corals, I've purchased many. But you need to be aware that if it is bleached, and you are going to take a chance on it, it will require more attention than a healthy acro.
Lastly visually check for red bugs and AEFW. They can be seen with the naked eye once you are familiar with what to look for.
Also please make sure you can tell the difference between tissue loss and bleaching. Many people confuse the two, but they look and are, two very different things.
HTH