Weight of the Rolesium (plat/ss) and sometimes rotor sound will give it away- especially if it's a 21J Asian version, as they have a tendancy to sound a little 'rough'. In addition, the YM is a hard one to get away with anyways- You have details like the CG's (which differ than most others in the Sports family), dial color/tone, font color, bezel color, bezel markers, etc. that all have to be spot on, or there's a very good chance that it will get called out. When your day to day involves handling and looking at these things constantly, it's easy to develop an eye to identify something that just doesn't look, feel, or sometimes sound 'right'.
This is why I stick with vintage MBW's. When genuine parts fit with little fuss, it's much easier to find and build a much more convincing piece.