I normally fly from Bellingham International Airport, USA (BLI) because I want to keep my passport pages blank.
There's no way to tell. Both the CBP and CBSA fully have the authority to stamp every time you cross the border inwards, neither country has exit checks but it's almost totally random.
Theoretically the CBP stamps once per validity so if you cross the US border then you get a stamp once and no more for the next three months if you have an ESTA or six if you move on a B1/B2 visa.
CBSA tends to stamp on every border crossing.
Land crossings, especially in a car tends to have less stamps than air.
This is a useful answer.
Based purely on personal experience and 4 or 5 trips per year and taking into consideration my passport is from the US....
I never get stamped crossing by car in either direction.
I get stamped flying into Canada maybe 50% of the time.
I never get stamped flying into the US.
I would hazard a guess that if your record shows multiple crossings the less likely you are to be stamped. But ultimately it is up to the agent you deal with each time.