Nobody is going to sell fake phenazepam and get away with it. Anything you could ship that might be mistaken for phenazepam by a user (based on effects) would be more expensive.
However, plenty of cut phenazepam is going around, often as much out of concern for users safety as out of a desire to cheat customers.
You may also be building a tolerance to benzos - i hear phenaz used recreationally can really build tolerance pretty fast, and a stunning number of people report that their phenaz "must have been cut", and then try taking some other benzo, and discover that they've managed to build up a hell of a tolerance.
All changes are weak, slow, and unremarkable for phenazepam, etizolam, etaqualone, and phenbrobamate. All from the same very reputable canadian vendor, who's now out of the business.
Another easy test:
Add product to water. Product should not dissolve, and should not want to suspend in the water either. Separate water, dry product, and re-weigh. There should be no difference in weight. Phenazepam is not water soluble, many cuts are.