dish soap, warm water & a scrub brush will do them no harm, marbles don't scratch easily unless you use some type of abrasive or chemical cleaner, diggin is fun anyway
Backlight them and look for a fiery glow like Arts, no glow no Moonie. The glass must be opalescent for it to qualify as a Moonie. Even under a low light the mibs should exhibit some opalescence if Moonies. Indirect sunlight (daylight) works very well as in Arts pic.
Original surface IMO if that's dirt trapped in the pontil folds I found it nearly if not impossible to remove from my handmades but if it's polishing compound as Art says water and a toothbrush will remove it with a little elbow grease, oh yeah "nice mib"