Does it have a VGA plug? If not, then you can rule out VGA res.
That leaves standard res, and mid res. Roughly 95-97 percent of all titles ever produced were standard res, and roughly 90 percent of all mid res games came in dedicated cabinets (no one really made many mid res conversion kits, because there is only a teeny install base of mid res monitors).
Do you have a dedicated cab for a mid res game? If not, then you almost certainly have a standard res monitor.
Mid res ones truly are rare beasts, I have owned over 40 games to date (and nearly 100 boardsets), and never had a mid res game. You are more likely to encounter a vector or B&W monitor than you are a mid res one.
A quick search of Klov.com only showed a handful of mid res games that were not dedicated units, and the majority of those were either incorrectly labeled, or had a dip switch to use standard res.