efficient way to verify one memory block is only readable?

When I did some experiments about vtable, and modified the virtual method address myself, I found error report : "Segment fault". When I see the /proc/self/maps, vtable is in the readonly text segment. I almost forget the fact that vtables have been stored in readonly segment. So I have a question : how to effectively verify one memory block is only readable or writable? I think it's not efficient to check all the parts in /proc/maps. Does anyone have another efficient way to do this verification? Thanks in advance. - mudongliang