You can tell the front ones are over grown by looking at them. The molars are a bit more tricky, since you can't see the molars you have to go by symptoms of their being a problem or take your chin in for an exam. Symptoms include things like suddenly not eating hay, pawing at the mouth, drooling, and dropping food out of their mouth. Most chins the vet can use a camera scope to check the teeth, but it depends on the chin, others require being sedated, in those cases they don't bother checking them unless they have a reason to. If your chin's teeth line up properly and they chew toys and eat a proper diet they should never end up over grown, unfortunately not all chins have perfect teeth, and things can happen (accident, something stuck in the teeth, etc) that can shift teeth out of alignment.
Once overgrown you normally need to get them trimmed. If only a little overgrown then sometimes getting them to chew more (chew toys if the front teeth and more hay if the molars) can help, but in most cases you don't notice until they have already gotten too long and need trimming/filing. If symptoms don't get better with just trimming and filing though, you will have to get x-rays taken, the roots can also grow, commonly from the teeth running out of room inside the mouth and can end up growing up into the skull and/or down into the jaw.