I guess the best way is using a Topographic Map for where you are. You can get them Free from topozone.com, just input your location, either UTM or Long/Lat from the GPS. You need to look at the Contour Lines to figure out your elevation or some rather large lakes might have the elevation.
BTW, most GPS units don't do very well when it comes to elevation, the manuals seldom publish the accuracy but it's terrible. I understand when the WASS system is fully functional, it'll be pretty accurate tho, like 3 meters.
None of the publicly available GPS's will do altitude worth a hoot--this is by design for defense purposes. The military uses a different frequency for altitude. It's possible that that your altitude reading will be even more off when the military is on alert.
I heard the reason for this is so that enemy misssles/pilots can't effectively use our GPS signals against us.
Most GPS units that I know of don't have an altimeter. They calculate altitude the same way they calculate your horizontal position. But mathematically, the altitude calculation is not as good as the horizontal.