Which header and lib files to use when compiling libpq-code?

I need to compile some code that uses libpq. For this I need to determine
the directories to use for the header and library files.

The machine I'm using has multiple copies of the files libpq-fe.h and
libpq.a. How can I determine which one of all these copies are the ones
that correspond to the Pg server that is currently running on this system?
(FWIW, the OS here is Ubuntu Linux.)

I tried the naive approach of using the "newest" versions of these files, as
measured by their modification times. This produced disastrous results.
The code compiled, but resulted in a program that failed in bizarre and
unpredictable ways, which strongly hinted at one or more silent segmentation
violations in the C code. I conjectured that this was the result of a
mismatched header or library file. Sure enough, the bizarre failures
disappeared when I used a different set of header and library files.

But this method of determining the correct header and library files is still
too fraught with uncertainty for my taste. Is there an approach to this
problem that involves less guesswork?

Thanks!

Kynn