Hi Ian,
I would imagine you could ask for a receipt that it's been opened, as per normal email. I've never bothered tracking them to be honest.
If you were really paranoid/excited about which customers had opened them and how many times etc, which would give you a good guide as to a "warm" lead, you could use something like spypig.com and then follow up with a call etc.
May be a bit OTT for window cleaning.
Cheers,
Huw