If you know the names of the companies, you could search the FCC database. Sometimes the licensee's corporate name differs from the DBA name (doing business as).
They might be 'buying airtime' from a company that specializes in providing repeater access for businesses. If that is the case, the license would be in the name of the repeater company, not the taxi company.
You could also search the old traditional taxicab ranges of 152.27 to 152.465 base; 157.53 to 157.725 mobile (15 KHz spacing), and 452.05 to 452.5 (mobiles +5 MHz), 50 KHz spacing. Taxi company systems usually do not repeat the mobiles, which prevents cabs from getting into discussions with each other, interfering withdispatchers are trying to air calls. The procedures and terminology are very different from any other service, and if you haven't been around them, may take some time to figure out (or ask a driver). Good luck!