You can check with BGTOLL via their website (VIGNETTE CHECK) or the BGTOLL mobile app by entering your license plate number and the country of registration.
The system shows whether a vignette is valid, start and end date, and the vignette type (weekend, weekly, etc.)
Related articles:
Do I get a confirmation email for my Bulgarian vignette order?
Do I need a payment receipt or print-out of the Bulgarian vignette?
Does the Bulgarian vignette activate immediately?