A pressure test of the engine is the best way.

All the external orifices are sealed and a pressure/vacuum test is done on the engine.

Typically people use roofing rubber or tire inner tube rubber to seal the exhaust behind the muffler and the intake behind the carb flange.

The crankcase is put under a slight pressure (10 psi) and watched for 1/2 hr as to its ability to hold the pressure. The front seal, the crankcase seams, and all the joints where the read block and intake manifold attach are bubble checked.

The same test is then performed with a vacuum (1 tor) and watched as a front seal integrity check.

The problem could be as simple as a bad gasket where the carb fastens or a leaking impulse line from the reed block to the carb?