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