Many times, you can tell if you're fully erected by feeling how hard, big, and tilted your penis is. The penis is much bigger, harder, and more rigid when it is fully erect than when it is limp. It can also stand straight up or slightly tilted away from the body.
The skin on the penis may also feel stretched and tight. Also, you might feel more sensitive or full in the groin area of yourself. Because everyone's penis is a different size and shape, what defines a full erection may be different for each person. If you are worried about your erections or think you might have erectile dysfunction, you should talk to a doctor or nurse to get a good diagnosis and treatment plan.