Simple compression test.

Remove the plugs, place a finger over the plug hole and turn the engine over.  The compression should be enough to "blow" your finger away, with a loud hiss.

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Whilst your'e at it.  Is the end of the spark plug wet with petrol?  If so, you've flooded the engine.  Turn the petrol off and open the choke, dry the plugs and try starting it again.

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If you can push the bike easily, with the plugs in, whilst it is in gear, then there is a good chance that you have no compression.  How can all cylinders fail at once?  Easy if your valve timing is out due to a broken cam belt / chain.