If one has a chance to look closely, they are quite different. Their major field identification features are as follows:
a. Brown-capped Woodpecker prefers coastal and cleared forests and in housing estates gardens, Grey-capped prefers primary and secondary forests.
b. Brown-capped Woodpecker's black in its upperparts is more grey-brownish and less contrasty then Grey-capped. Generally the Grey-capped will look blacker than Browned-capped.
c. Brown-capped Woodpecker's underparts lacks rufous wash, while Grey-capped's rufous wash on breast and belly is evident in the field.
Here are images of both species, when you compare them side by side, these features can be easily spotted.
Photodocument of Wild Birds of Borneo.
Standard References for my blog.