Easiest way to check for a leak is to spray a little brake cleaner onto the junctions of the engine while it’s warmed up and running.
Just tiny squirts all around the head/cylinder junction. Cylinder/case junction. Intake manifold base and where it clamps on the carb. Exhaust header flange.
If you make sure you don’t overspray directly toward the mouth of the carburetor while you spray any of the above mentioned spots and the engine rpm changes, that means it’s sucking in the flammable “fuel” (brake cleaner) and changing the combustion mixture. So, there’s a leak there.
Sometimes you can hear or feel a leak. Maybe even see a spot where oil is dripping or spattering from it.
There are crank seals that can go bad. One is on the clutch side and it might be hard to know if it’s bad. There were a couple recent posts where trans oil was coming out of vent holes on the upper rear of the engine. I suppose that could be how you’d see that seal leaking.
The other crank seal is behind the flywheel. If it’s clean and dry, it’s probably not leaking. I’ve never shot flammable stuff onto my ignition. Maybe someone else can chime in there.
That’s a lot of words, but only a minute of actual work.