As it happens on the ground, motorbikes are not allowed on the expressways, so this is a non-starter. Legally, it is ambiguous, yet it seems that the practice is no bikes allowed.

For the other tolls on regular roads, bikes don't pay. Just go around the booth on the side. However, the Chinese often set up police checkpoints at the same location of the toll booths. Sometimes they will check bikes going through, in which case you need to know which tolls also function as checkpoints and how to avoid those.

It's a booth-by-booth or checkpoint-by-checkpoint specific scenario so it's best to cruise up slowly to the booth first, stop in advance to see what's going on, and then plan your route for how you'll get through. For example, sometimes you go around the side, sometimes you cruise slowly, other times you blast right through, other times you have to completely avoid the checkpoint using back roads.