There’s an old trick that Jeff Renner used to post that describes filling a plastic bottle at bottling time, squeezing the air out and screwing the top back on. As the beer carbonates the bottle will expand and get more firm so you can observe the level of carbonation taking place.
Also, if you mark the fill level on a few of your bottles with a sharpie, you can actually witness the level of liquid increase as the beer carbonates.
But I haven’t bottle-conditioned beer much in years…
I just tried a 12oz of my Al’s Celebration Ale (see sig) yesterday, and it was definitely carbonated after ten days, but not quite ready yet. Since that was my first batch, I was mostly just very happy to discover that it came out good! Note that these are being carbonated using 2/3 cup* of corn sugar, and the temperature is pretty steadily about 70F. It was obvious that they needed more time to condition though, so be patient.
I will be waiting until at least 20 days before trying another 12oz. I just learned in this thread that bigger bottles take a bit longer, so I will wait a little longer on those.
*we weighed it, not measured, but we weighed out enough for TWO batches, and what I actually used was divided by eye, so it’s probably not entirely precise, but it’s pretty close.