Hi,
Given the resolution you are wanting to see, say, departures from perfect flatness of ± 1.0 mm or so at the best, I'd use an engineer's straight edge
http://www.toolstore.com.au/estore/style/agp12.aspx
and a set of feeler gauges
http://www.toolmart.com.au/measuring-tools/rt3100.html
You lay the straight edge across the base in various radial positions and use the feeler gauge to measure gaps beneath the straight edge when it is in a stable location.
Cheers