Clobbered MBR partition table

[George Mitchell]

My laptop has a hard disk I partitioned, whoops, I mean sliced, into
four slices when I installed 8.2-STABLE on it a couple of years ago.
The first, third, and fourth slices I reserved for future experiments,
and most of the space went into the second slice where I installed
8.2-STABLE.  Time went on and the second slice is currently running
9.2-STABLE, and I installed 10.0-PRERELEASE on the first slice late
last year.  But mostly I have been booting off the second slice, which
means pressing enter at the initial F1/F2/F3/F4 boot prompt.

Then last Friday I was preparing to update the first slice to the
latest 10.0-STABLE.  Things were going well until I rebooted and
typed F1 at the boot prompt.  I immediately got a second prompt
offering me the options of second disk or PXE.  At this point the
machine was unbootable, as whatever I typed would cycle between the
F1/F2/F3/F4 alternatives and the second disk/PXE alternatives.  I hit
ctrl-alt-delete and got told there was no bootable disk.

So I got a new disk and plugged in into the laptop and started over
again.  (My first attempt was with a 10.0-RELEASE memstick image,
but that's a subject for another day.)  Out of conservatism, I have
installed 8.4-STABLE on the new disk.  Then the first thing I did
was to hook up the old disk through a USB adapter and dump it with
dd to a backup image.  That's going to be finished in a couple of
hours, at which point I hope to poke around on the old disk and
repair what I assume is a clobbered master boot record partition
table.

My question is:
What's the best tool to help reconstruct the partition table?  I
think my problem will be mostly solved if I can find the BSD labels
on the old disk (which, by the way, has not exhibited any I/O errors
during the "dd" backup process).  Do BSD labels have a recognizable
signature?

Thanks for your help and attention.                       -- George