I usually wipe my ESD mat down with the ACL Staticide spray. I also clean the floor up in the area with it. It appears to be basically non-toxic according to the SDS and I've found it's very effective (sprayed down a 'fleece' blanket and the static is all gone.)
The crude way of measuring surface resistivity in ohms per square would be to use a couple square foil electrodes weighted down with two flat (or slightly convex) heavy objects (spaced apart the same distance as one side of the square). Place this unknown resistance in series with say a 50 volt DC supply and a common 10 Megohm input impedance DMM. Record the indicated voltage, calculate the unknown resistance based on the understanding that you have formed a voltage divider of the unknown resistance and the 10M of the meter. You should be able to reasonably discern resistances up in the several Gigaohm range without resorting to dangerously high voltages this way.
The good quality mats have a dissipative surface (I think generally 1Gohm+) (usually a light colored rubber) on the top and a conductive surface (<100K ohm) (usually a black rubber) under it. Thus you get good conductivity to the grounding point, but less chance of disturbing measurements when in contact with the surface.