Peter,
You look for the abnormal. You try to find what appears out of place.

In order to do this with any degree of expertise, you need to look at chips and more chips and more chips. This is what gives you your base for comparison.

It has taken me a few years and a lot of dealer binders but it is not that difficult.

If a chip "doesn't look right" or "feel right" go with your first impression. It generally will be correct.

Best, Jim