Sablesurfer,
Go to
http://www.accuridewheels.com/safety_manual.asp and click on Section IV. It has some good pictures and text to help you identify if you are running one of the several types of older model split rims. ( There are modern rims that some people call split rims, but they are actually multi-piece rims that are designed completely different than the classic "widowmakers" )
Another way to check is to look at the valve stem. If it is press fit into the rim, it is likely tubeless. If it is poking out through a hole or slot slightly larger than the stem, it is running a tube. Some tubes also use a knurled nut around the stem to tighten it to the rim.
Also, most modern tires are classified as tubeless and are stamped as such. This doesn't mean that you can't use a tube and mount it on a tube type rim. (Heck, if you wanted to, you could pull the valve stem from a modern rim and run a tube. I've done it in a pinch.)
The problem with using a tube at highway speeds is that it generates more heat than a tubeless tire alone.
Post up some close up pics, that will help us help you too.
