Checking for old transaction snapshots

I have a job that does a big batch delete/insert. I want it to vacuum (or
probably cluster) when it's finished. But I figure I should sleep for a while
before doing the vacuum to be sure there are no old transactions still
running.

Is there a simple query I can have it do against the system tables to check
for any transactions older either than when the batch delete finished?

I'm also interested in verifying that I don't have the problem of the
front-end application issuing a BEGIN as soon as a script ends. Ie, starting a
transaction that will lie idle until the next page hit that process handles.

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greg