As Dorothy mentions, there is a "point system" in use. The ACEP method is also popular. I'm more inclined to use ACEP because the point systems I've seen tend to derive from separately reportable CPT and adjunct services (e.g., 1 point for a patient getting an x-ray, 2 points for one getting a wound/injury closure, 1 point for an IM injection).
Honestly, in looking at how facilities determine their ER E&M levels, I find a great deal of inconistency. From some of my auditing jobs, hospitals tend to score everyone 99284 and 99285. They have an even more difficult time pointing toward how medical necessity supports that level. Even so, without explicit, tried and sound methods, this is what payers have to contend with . . .