OK.
How can I tell if one is good. I hired a guy last year to clean up my computer and save some data and 800 dollars later I find he was marginal at best. And he came well recommended.
If you walk around the area you live with a sandwich board saying "Free Republic Rules!" for 48 hours I will fix your systems for free.
YIKES! Therein lies the problem.
You are doing the right thing- The best way to find the good ones is by word of mouth. One fair sign is if the one recommending is running a white box (non-brand name), and doesn't have much problem with the machine, viruses, and etc...
Very few of my clients run store-bought machines (even though I don't build them, I farm that out), and it is unlikely for me to see a client much more than their annual tune-up...