Hi,

You need to do this using a 4-point probe and a parameter analyser.
This is the only way you can measure bulk resistivity whilst excluding
the contact resistance at the probe/substrate interface. To improve
current injection to the wafer, you could consider coating the surface
in a noble metallic film, and pattern this accommodate the 4-point
probe arrangement. If you need further elaboration on the method, have
a look at any textbook on electrical characterisation or do a web
search. The parameter analyser is basically a very high impedance
voltmeter, sensitive to pico-ampres. Such units will also come with
manuals that advise on the procedure for conducting 4PP tests. HP even
provide their manuals online I believe.

Hope this helps.

Good luck!

Michael