How can I tell if I was dosed with rohipnol?

Only after. If you are in a circumstance, offered a drink by a relative stranger, and wake up the next morning naked in his bed, then that is how you can tell. It adds no taste to a drink you might be offered. And once ingested, you are stuck with the effects. Always be careful, be with trustable people, try not to be a loner and control what you ingest.