They mass product the us kill-a-watt.  They are pretty cheap I actually had to get one today.... some day I will find a project with my old one connected to it but I could not for the life of me find it in my normal places.

One option is very advanced PDU's can tell what it is.  I got one thanks to Phil pointing me to a sale on ebay, but I never did get the networking going (yet another project ).   The 240 watt meters are a little higher.  I bought a fluke meter once when I thought I need it.... once I figure it out I should be able to tell with a clamp that just goes around the cord.   So that is my goal is to get it figured out eventually.

And on a side note I have enjoyed seeing some PC's brings back so much memories. And love all the options you have with them.  So much personality to each one.