Summarize this article:

The single most noticeable symptom of blood blisters is bumps that appear just below the skin. If you notice your cat developing bumps, you should investigate them. When looking at bumps, look to see if:  The bump is raised slightly. If there are multiple bumps in the same area. One of the hallmarks of a blood blister is its reddish color. The blood blister is red because it reflects the blood that is pooling just below the skin. In the end, the darkness of the bump is one of the best indications that it is a blood blister and not another type of blister or growth. Larger blisters or ones in problematic locations may cause your cat pain. However, it is important to note that not all blisters are painful and blisters are not the only type of growth that may cause pain.  Try to notice if your cat experiences pain when you or they touch the blister. Blisters around the neck or in your cat's ear may cause your cat significant discomfort. Blood blisters on the pads of your cat’s feet may cause your cat to limp.

Summary:
Look for a bump. Notice that the bump appears dark red. Watch to see if your cat is in pain.