For most routes there are only one or two types of train that routinely operate services. So you can just tell us what route (and operator) you'll be using and someone here will know.
For certain operators, on the day
www.realtimetrains.co.uk will be able to tell you the exact unit (train) that is allocated to run your service. This includes CrossCountry, Northern, LNER, ScotRail and Caledonian Sleeper at present. You could look at the same time service as you're intending to take today or yesterday to see what train was allocated.
If there are reservations available (with specific seat allocations), then
www.TrainSplit.com will show you a seating map which will show the layout of the train the operator intends to run. The number of carriages or coach layout may give away what kind of train they're intending to run if you cross-reference it to the relevant operator's generic seatmaps.
Finally, you could use apps like On Rails, or other sites that make use of the Darwin data feed, to see how many carriages the service is shown as having. However this doesn't necessarily definitively narrow it down and the data isn't available at all stations or for all services.
