An easy way to tell how much potential in general you have is to measure your wrist circumference (measuring around the bone that sticks out). If you have more than 7", the general consensus is that you can build 21" arms. (Of course there have been a few pros who had smaller measurements, but they were also on huge amounts of anabolics, etc.) You also have the "athletic" look which is pretty much being a mesomorph. Do you find you gain fat easily if you don't watch what you're eating? If you do, then that is the endomorphic part. Most people are a combination between two types.
I have exactly 7", and I was 130 lbs when I graduated high school and up until I was 29 years old. Training my ass off and eating as much as I could I got to 150 at 30 years old. That makes me mainly an ectomorph, but I also had low testosterone so when I was put on HRT, I gained 30 lbs in 3 months. I've gained steadily since then to 210 so I have a bit of mesomorph in me as well.