Exhaust leak definitely causes backfires. Usually only when you have let off the gas. The backpressure sucks fresh air into the leak and ignites in the exhaust pipe.
I've heard of folks attaching a shop vac to the exhaust and then blow air into the exhaust, then spray soapy water on the exhaust and look for bubbles.
I've heard of folks attaching a shop vac to the exhaust and then blow air into the exhaust, then spray soapy water on the exhaust and look for bubbles.