Checking database version

Is there a reliable way of verifying version of the database by looking at
the PGDATA directory?

Let me explain why this is important in our case. Originally, our product
utilized version 8.1 of Postgres. We have since moved on to version 8.2.
Our existing customer base will need to be upgraded to version 8.2 as well.
There might be legitimate cases where our customers might be running other
software products on their systems, which utilize earlier versions of
Postgres, therefore we cannot uninstall earlier packages and install ours
indiscriminately. Our install scripts must reliably determine if the
database files related to our product need to be upgraded or not, before
installing Postgres 8.2 package, performing dump, etc.

What I'm looking for is a sure way of determining version of Postgres based
on the files contained in a specific PGDATA location. Is it possible, and
if so how can it be done?

Thanks in advance,

~george