There are a lot of taxis in Bulgaria! They have checkerboad stripes on their sides. Usually they are yellow, but more rarely other colors, such as gold or lime green.
There are different taxi companies in different Bulgarian cities and they often show numbers or other signage on the front or sides of the taxi indicating the company. By knowing these we can identify the city.
The spreadsheet below shows some of the most common signage in different cities, which are listed by decreasing population size. There may be other signage not included here and, since taxis travel, it is possible to see the signage associated with one city in a different city.