The kWh on the power bill may give you an idea, but you can only calculate the average demand, and not peak current. Has the meter recorded peak current demand? If not, that's your first step. The best way is to hook up a power quality analyzer (or similar) on their service and leave it there for a year. That will tell you peak demand, as well as a wealth of other information like power factor and transients.

Even this analysis doesn't give the true full picture; I will never design to more than 80% capacity to allow for unplanned surge.

A spot check with an ammeter may give you an idea of capacity, and in some cases may be good enough (if they have a 3000A service and 100A of load, I think you're OK.) It may also give you an easy "no" answer if they're drawing 1200A on a 1500A service. But if it's in between, you must consider that HVAC load can be more than the rest of the building put together, yet only shows up on the hottest/coldest days of the year.