How can I tell if my manhood is getting bigger?

Hi Adam,

I'm sure you're growing.

It's hard to tell.  You see your penis a lot so it's hard to notice it growing.  It's like looking in the mirror, you don't really notice changes day to day but if you looked at a picture from a couple of years ago you do.

It can grow into your early 20's Adam. 

Hope it helps.

HI Adam
As Dan stated in his reply, the changes would be hard to notice as you see it so often.
Being 13 and still having a small penis is very common and nothing to worry about as you have many years of growth and development ahead of you.
If you feel you really need to keep track of the gradual changes as your penis grows, you could look up proper ways to measure the penis and once you have learned how it is done.
You could perhaps take monthly measurements to keep track of your growth.
Everyone grows and develops at different rates and times and depending on how far into puberty you are, your turn will come soon enough as well.