Measuring girth can be difficult IMO because the urethra that tube on the underside of your penis where the urine comes through, is easily compressed when you measure. So its hard to get a real accurate read, in my opinion anyway.
For me the biggest way to tell is using my first pumping cylinder. It used to be large for me, now I barely fit in when I go to use it. Its almost like it looks like my dick didn’t get thicker, just that the tube shrank! :)
So, slow gains, like children growing…you don’t really notice, it just kind of sneaks up on you.
So find a tube or something like a toilet paper roll tube and see how it fits. Then come back to it in about 6 months and see if it still fits, or it you can’t even get into anymore. Thats how I like to keep track of girth.
For shorter term measurements, I like to use a string or cord, and just make a mark on it where it wraps around once and touches itself. Make an new mark on it every time its longer. Then you can lay the string out and measure it with a tape measure. I think that makes it more accurate.