Compute hash of a table?

Hi All

I need to determine whether the content of a given table has changed from
the previous time I've checked it. so, my initial idea is to calculate a
hash value of the content of the whole table, may be with custom aggregate
function or something.

My question is:

Is it possible to get the last time(stamp) when the content of a given table
in pgsql has changed???

Thanks!
Pete