Can this be used to check a concrete subfloor for divots and peaks?

Great question! The short answer is yes, the CG100 Cutting Guide can be used for that task, but please make sure the connecting piece is tight and secure - you want to ensure there is no bend in the middle between the 2 pieces. Alternatively, you can also check a subfloor by putting the Cutting Guide on its side just like a straight edge to see the gaps. Hope that helps!