There are several signs of a bad haircut--you can see the line of demarcation between the top and sides, the hairline is a mess or there are long pieces of hair around the ears. It only takes a few minutes, at the end of a haircut, but trimming the hair around the ears makes a huge difference in a haircut.
Notice the long pieces of hair around the ear that you didn't get during the haircut. This is common because they tend to lay in different directions and are hard to get.
Run the clippers along the top of the ear and around to the back. Make sure you're going against the direction of the hair, so that the clippers pick up the longer pieces and trim them.
Fold the ears down. You can hold them if you have a free hand. If not, as the man who hold his ear down for you.
Use the scissors to trim the long pieces of hair. Make sure to trim them the same length as the surrounding hair.
Angle the scissors in any direction you want, as long as you nip the pieces of hair and not your client's ear.
Comb the hair in an upwards direction. Hold the comb flat against the head.
Trim along the comb, getting the pieces of hair on this side of the comb. Make sure that it's blended well and not noticeable shorter or longer.