I found a good video, which explains eight ways to tell if someone is using steroids:

This video somewhat addresses the increases in strength that are attainable by someone who is not using steroids, versus someone who is using steroids.

For instance, this video creator explains that it took him four years of weight training to increase his bench press from 200 lb to 300 lb, without using steroids.  He states that when you see someone increase their bench press by 100 lb in a matter of months, that this is a definite sign of steroid use.