You can't really test a manufacturer's service before you buy, but you can ask other owners about service. Try to find an owner in your geographic area, who is using the same rep you'll be using if you buy the copier. This is important, since a company's service is only as good as the rep. The next question you need to answer is what exactly do you want your new copier to do. Increased productivity closely followed reliability on the list of things our interviewees said they sought in new copiers.
Productivity is a function of speed, of course, but also other factors, such as ease of input.
Basically, you can trust the sales literature. Though it's not guaranteed to be accurate, vendors know they can't stretch the truth too far. Or, of course, you could take a stopwatch to the copiers you're examining and time them. Don't stop at speed alone, though. Another key factor in productivity is the number of copies that can be made each time you feed the original into the machine.