Looking at RTT how will I know which services through Worcester will be IEPs?
It's IET - InterCity Express Train, the GWR brand for the units, as opposed to IEP, which is the Inter City Express Programme, which was the government programme to acquire the trains.
Is there a particular day you are planning to travel? And time of day?
If you want a nailed-on HST at the moment and don't mind an early start or late finish, the weekday 04.47 off Hereford and the 05.11 off Worcester are still HSTs, as are the 19.22 Paddington-Hereford and the 20.22 and 21.48 to Worcester but I seem to recall being told that overnight IET stabling at Worcester and Hereford will be starting at some point in September, so these services may switch to IETs in a few weeks' time.
The 06.42 from Hereford to Paddington was an HST until yesterday but I have just noticed that the empty stock from Bristol is shown on realtimetrains from Tuesday as coming from Stoke Gifford IET depot rather than St Philips Marsh HST depot, so looks like that is the next service to go over to an IET. I am due to be travelling on it to Reading on Tuesday, so will be able to confirm if that's the case.
In the evening peak, the 16.22, 17.22 and 17.52 from Paddington are all IETs now.
I think both the off-peak trains from Paddington to Hereford and back, the 08.22/12.09 and the 10.22/15.14 are still HSTs as well but the diagrams are a bit of a moveable feast at the moment, so there are probably no guarantees. The 07.50 from Paddington/10.59 from Malvern is still a Turbo, as is the morning halts train to Didcot, but most of the other weekday off-peak services should be IETs (the first to reach Worcester in the morning are the 05.47 and 06.52 off Paddington, forming the 08.25 back from Worcester Foregate Street and the 09.54 from Great Malvern.
At weekends, there seems to be a preponderance of IETs now - it would probably be easier if someone can tell you which Cotswold Line duties on Saturday and Sunday are still HSTs.