What is steroids?

From what I gather, it is something that blocks certain passages, and advances other passages, essentially mimicking the natural testosterone hormone. Therefore when somebody consumes injects or uses a lot of steroids, they just end up looking over-masculine… or it looks bizarre because their testosterone levels are *too* high?

Something I am then curious about is this:

What are the natural physical and physiological traits of a man which actually signals and exhibits truly high testosterone levels?