pre-Vacuum statistics

Greetings,
All the recommendations I can locate concerning the use of (the various
flavors of) VACUUM suggest running it at regular intervals. Is there any
way, for a given table, to discover how many/what percentage of rows are
likely to be VACUUMable at a given point, so that some kind of
threshold-based VACUUM could be done by an application? We have several
tables that undergo bursty UPDATEs, where large numbers of transactions
occur "relatively" infrequently; and others where the UPDATEs occur at
regular intervals (a few seconds or so).

Thanks for any advice!

                     Regards,
                     Rich Cullingford
                     rculling@sysd.com