The snag with satellite navigation is that it is mostly very good indeed, occasionaly not bad, but on rare events - wrong. I just saw some measurement statistics from a receiver in London for 2006 - 99.5% of the time, the horizontal error was under 4.9m but 99.9% of the time it was under 3.6 km.
Some receivers do give an indication of Dilution of Precision (DOP) which is caused mainly by a poor combination of satellite positions. Some provide an indication of signal (strictly signal to noise ratio) strength per satellite, as weak signals correlate badly and cause measurement errors. Some (expensive ones) provide an estimate of multipath effects, which is the most likely cause of large errors in a ground vehicle installations. Then there are those nasty ionospheric induced errors, which WAAS reception should help to remove, or you need a dual frequency receiver, and then your local troposphere, which injects a bit more error.
One saving grace is that most of the errors are transient.