My understanding is that grazing 'leases' are actually fairly rare Province-wide, however some areas, such as around Kamloops, seem to have a lot of them.
Some are posted and some are not. However, I believe the same rules of private land apply, that being if the land is 'enclosed' (ie. fenced, posted and/or cultivated), then you need permission to access.
It is best to just check the link that Doostien posted above - the crown leases are clearly shown.