You'll definitely have to watch the chainstay length. CX bikes with more of a "commuter friendly" build (eyelets for racks and fenders, slightly longer wheelbase, more BB drop) would be better than the more race-oriented machines.
One way to gauge this visually is to look at the size of the gap between the front of the rear rim and the seat tube (I say rim because this takes tire size out of the equation). Racing cross bikes will have short chainstays for responsive climbing and a shorter wheelbase, and this gap will be pretty small.

As JeffS mentions, the total geometry of a given bike will vary between styles and brands, so it is helpful to be able to throw your leg over several different bikes to get a feel for size ranges that might work.
Bikes with a sloping top tube design can be deceiving. The seat tube measurement may be quite small when compared to that of a conventional (horizontal top tube) frame with similar geometry, though both bikes could very likely be identical in all other aspects.