How do you know when to shift on a dirt bike?

Depends if its 2 stroke or 4 stroke. A 2 stroke bike you can tell by the RPMS which they will be real high which your bike will be very loud. Also if you have a 4 stroke bike you can shift anytime just 4 stroke bikes have torque all the time. so it really depends if your racing just cruising or whatever it all depends on what you r doing.