Once you've found out the four-letter ID ("train description") of your train, you can also cross-check with the live signalling displays provided by Opentraintimes (https://www.opentraintimes.com/maps/signalling/lec1), Traksy (https://traksy.uk/live/M+39+EUSTON+-29) et al.

Those will show a copy of the train describer data, which is the same thing that the actual signallers use to keep track of which train runs to which timetable, so they (or the computer, where available) can route it accordingly.