efficient way to verify one memory block is only readable?

[mudongliang]

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