Find something like a night site for a handgun, a milspec compass or something with some radiation and take a reading with each meter. If they all same the same thing at least they are all on par with each other. It might not be calibrated, but it's a good sign if they all give the same readings. Or just break down and pay to have them checked. I'd send one or two at a time, some can keep your hands one the others.
OK... before there is a lot of erroneous information gets unleashed here, take some good advice. I'm a Fema certified Radiological Mointoring Tech.
If you have the CDV - 700 (low-medium radiation), put fresh batteries in it, put on the earphone, open the Beta-window on the probe by turning the probe shield, turn the switch on to 1X and put the B-window next to the tiny bump on the side decal of the CDV that says "Calibration ource". If it's working right, the needle on the meter should climb toward the right side fairly quickly, and you will hear rapidly accellerating "clicks" the closer to the source that the probe gets. Move it close to the source and away from it to test the difference. This is by no means an indication of correct calibration, most of the calibration sources are from the cold war and only have half or less the strength that they did back then.
If you have the CDV - 717 or CDV - 715 (high radiation), the only way to test to see if it worth keeping is to put fresh batteries in it, turn the switch on to "circuit check" and watch to see if the needle makes it up into "Circuit check" range on the meter... sometimes you have to put it on "Zeroing" and turn the zeroing knob to set the needle to zero first. There is no legal radiation calibration source strong enough for you to test these with except for a licensed testing lab. The one I use is here: Civil Defense Radiation Detectors & Detection Meters FAQ & Geiger Counters radiation safety!
These people charge about $75 to calibrate and test one, -this is something you can't do yourself. I don't know of any radiologist or dentist willing to risk their license (or thier health) to test one...
IF the circuit check is OK, that's still not a sign that the counter is calibrated for accuracy. If you wait for WTSHTF, any reading will be guessing.
BEWARE of what you bid for on E-bay... there are a lot of sharks out there to take your money. I know a lot of people who have gotten ripped off. Make sure that the seller has a 7 day return policy, to enable you to do some primary tests as I have described.
PS... if you need a testing source for a CDV - 700 (only) because the calibration decal has been removed, you can use a Coleman lantern replacement mantle... the older ones were slightly radioactive... enough to make it click and the needle move. If you have Fiesta ware, the red colored ones (Uranium in the glaze) very often will make the needle go off scale - you can find these at antique stores and second-hand shops. Old Wesclock clocks that have dials and hands that glow in the dark will literally buzz the counter with the Radium paint of the numbers and hands. You can find these, plus radioactive ore samples advertised on ebay for this purpose.
It may be urban legend, but I'd heard that a lot of employees in the old Wesclock plant had wound up getting cancers of various types years down the road from working with the radioactive paint day in and day out. Not enough to cause any problems to family with one clock on the bed side table, but the amount of exposure in the plant was something different all together. Just what I'd heard anyway.
Yes, you can't buy anything with Radium anymore, unless it's antique.
Those red Fiestaware dishes will sure buzz a Geiger counter too! I don't think they make them anymore either. There isn't enough radioactivity in the uranium laced glaze to cause harm though, -but I wouldn't want to have them next to my skin for very long periods of time.