There's a little script that Martyn Smith wrote (and I hacked on) to list for each node when puppet last ran.
Example output:
nigel@freud:~$ sudo puppetwhen
10 minutes ago: dnsadmin-master.wgtn.cat-it.co.nz
13 minutes ago: myportfolio-school-nz.wgtn.cat-it.co.nz
13 minutes ago: myportfolio-ac-nz.wgtn.cat-it.co.nz
14 minutes ago: mysql.wgtn.cat-it.co.nz
14 minutes ago: pg81.wgtn.cat-it.co.nz
15 minutes ago: dnsadmin-slave.wgtn.cat-it.co.nz
16 minutes ago: dnsadmin-frontend.wgtn.cat-it.co.nz
16 minutes ago: dnsadmin-database.wgtn.cat-it.co.nz
19 minutes ago: dnsadmin-dr.wgtn.cat-it.co.nz
26 minutes ago: mahara-trunk.wgtn.cat-it.co.nz
26 minutes ago: mahara-1-0-stable.wgtn.cat-it.co.nz
26985 minutes ago: mahara-0910-pgsql.wgtn.cat-it.co.nz
26999 minutes ago: mahara-10-mysql.wgtn.cat-it.co.nz
27011 minutes ago: mahara-10-pgsql.wgtn.cat-it.co.nz
35681 minutes ago: mahara-dh-maketest.wgtn.cat-it.co.nz
35686 minutes ago: mahara-0-9-stable.wgtn.cat-it.co.nz
It's written in perl and uses a hardcoded path to the vardir. Suggestions from IRC include rewriting it in ruby so the vardir is easier to find.