Most compilers provide a facility to save the Preprocessor output to a file; with Keil C51, it's the PREPRINT command-line option.
Conventionally, this creates a file with a .i extension.

If you have unexplained errors, or suspect that your macros might not be quite working properly, I recommend that you should check this preprocessor listing!

And if you're really serious about finding little gotcha's and non-conformances, run lint on your code and see what it whines about!