One method, perhaps the most accurate way to test the crankcase seals for integrity is a compression / vacuum test, I blanked off both exhaust outlets and made a blanking plate for the intake manifold, with a hose tail joint welded on. I had access to a Stihl 2 Stroke compression / vacuum test kit to allow testing of seals in both directions.

Or, as many folks do, spray carb cleaner around the edge of flywheel, if the revs rise on tickover the seals pulling air in at the very least.

My bike just wouldn't start from cold, but the problem manifested itself over time. When I first got the bike it started fine, then after a few spirited rides with a dry / hardened seal, it just got gradually torn up. To a point where the bike just wouldn't start without easy-start spray. After it had finally fired, and had air / fuel mix pulled through the cases and had warmed a little, it would start sooo easy. Initially I checked everything else to no avail.

Let us know how you get on and if you need any help.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30