There are a number of indicators (Google has so many of these) but a few that I go by are: able to stay dry for a reasonable amount of time (2 hrs+, during a nap, etc.), able to communicate they need to go (sign language, motion, running to potty, or verbalize), and can pull their pants/underwear down (doesn't really seem "worth" it if they CAN go potty, but risk having accident after accident because they can't pull their pants down. Again, there are others, but these are particularly what we went off of for my first. I'm no where near potty training my 17.5m old yet.
@Grace Yes he hasn’t been writing his diapers during night.
If you notice they go through out the night without a wet diaper.