Detect Installed Version of WSE

I asked this question a few months ago but there have been two service packs
since then.

Can anyone recommend a reliable mechanism for the detection of WSE1/WSE2
being installed on a PC?

For example, are there certain installed files and/or registry settings that
exclusively indicate WSE2?

Currently, I check the GAC for file versions. However, I'm not sure how to distinguish Service Packs (I don't/can't have multiple Service Packs installed).

I intend to implement the check in VBScript.

Further, how can we distinguish between an installation of:

WSE1 (RTM)
WSE1 (SP1)

WSE2 (TP)
WSE2 (RTM)
WSE2 (SP1)
WSE2 (SP2)