For Mondays to Fridays services, you can look at the FCC PDF timetables on their website - if there is a 'star' at the top of the column it is a 377 service, because they have disabled toilet facilities. So far as I can tell, FCC treat the 319/2s (ex Brighton Express), which have disabled facilities as just another 319 and I have never known them be used as 377 substitute, other than at times of service disruption.
There are a few 377 diagrams at the weekend, but these are not shown in the PDF timetables in the same way.