I gave descriptions in the wiki. If a game seems to play just like the arcade, it's Playable. If the sound or action is distorted or laggy, it's Problematic. If it doesn't work at all or especially if it crashes the app or device, it's Broken. Ideally we would use %, but some games work better with some frameskip than others do, and i don't want to limit testing to those who are willing to check emulation speed %...

There's probably a way to see it in-game, but the only way i know of is by running it from the command line and looking at the output when you quit. It will give you a percentage that reflects the actual emulation speed vs. target. It's sometimes possible with -autoframeskip (which is set by default) to get much less than 100% emulation speed and still have a play experience that is not noticeably different than the original.