Site visit. Check their prepress area to make sure they have modern equipment.
CTP, PDF workflow.
If the stuff looks old, don't touch 'em. Lots of printers still use RIPs from like 1995 and then complain that your job doesn't process properly.
I'm sorry, but that is just ignorant. If you run into a case like that you clearly don't have a quality shop. A good shop will give you exact guidelines for how to submit work and will be able to explain exactly why your results are what they are. Not that things are not better than they were 10 years ago, but there is no reason to expect to get low quality from a 90's rip. Our university prints very high quality jobs (coffee table photo collections) and they still assemble generated film by hand.
What is important is what they produce, regardless of the equipment. The technicians and management are more important than the imagesetter, rip, scanner, etc. If it's a small locally owned shop the owner should be anal about details and procedures. We had a local company, before dtp who made all teh techs use straight blade tape dispensers for better draw-down, that's the kind of shop that will produce quality results. Also, do they profile their monitors (do they even know what an icc profile is), do they profile their imagesetter and press, do the pre-press ops know what they are doing, can they make quality seps?
Many artists don't understand printing very well. Many of them can't or don't take into consideration the print process or the capabilities/limitations of print. If you have a good artist and a good technician you can get high quality results, if you don't it's a crap shoot.
Then again, do you care, are you doing coffee table books or business cards.