Tungsten S4 fixtures don't have a serial number but there should be a small square sticker on the yoke that says something like 'ETC 06/09' which is the manufacturing date.
If you have one of our LED fixtures with a zoom lens on the front then let us know which type it is and we can let you know where to look.
OK, this is a tungsten fixture so as I said above these aren't serialised in our system. The closest would be to look at the manufacturing date but this doesn't really reflect when it was sold by us. Is there a reason you need to know?