GetIsModified is method used to do this .. but limitation is that it return true also if user do nothing with model but creo does .. for example some calculations during regeneration - geometry or props of model like params, material etc. may not be changed so use this method carefully.
WatchService may be usefull but you need to be careful too because Creo handle model files not in the way you one would expect. I am not sure what was the situation when I tested it but as I remember I tried to rename modle so watching files in whole directory and Creo did like 3 or 4 file operations where you would except only one - it was like creating new file, copying the file, deleting old file and renaming new file (as I say I cannot remember details, but it surprised me)
I think your only way is to do reverse engineering how Creo is handling model modification and file changes and try to fit the logic to for your application. Maybe make some combination of both GetIsModified and file watcher to achieve what you need.
Hope this helped.
