Most compilers have an option to disable all their extensions.

So your first step should be to use this option in the compiler from which you're porting the code. Thus the "original" compiler will identify everywhere that you've used its specific extensions.
You can then go to the "destination" compiler's Manual, and look-up the corresponding syntax for all those features

For Keil, see http://www.keil.com/support/m...extend.htm

Note that this just identifies syntax extensions - it won't help you idetify where you've relied on other things like calling conventions, data representations, etc...