Compression gauge should read 4 to 6 PSI at TDC and hold the compression, if you notice the PSI reading slip down then you have a problem. the engine will still run but not without flame out..

The engine weight method is crude but very effective and is widely used. as stated by Scrapheep

The other method is to just run the engine till it dont wanna run no more that the best way unless you are racing and need a reliable motor..