Throttle wide open. Kill switch pushed. Keep kicking until the gauge stops moving. Either warm or cold engine( the difference in readings is marginal). Leave the exhaust connected. Compression on a 125 should be above 150 at least. Don't rely on compression testing to check top end woes. Tear down the top end and measure the cylinder, piston and ring(s), ring end gap, and check the condition of the rod and rod bearings. Even when these parts are worn out, your compression can still be quite high, giving you a false sense of security.