We have a "Targeta de Transporte" license and are fully insured for hire and reward. Be warned, this isn't always the case..
There are numerous illegal Companies on the Costa Blanca openly advertising on the internet as legitimate taxi/transfer companies. There are also apartment owners & letting agencies and golf tour companies offering cheap transfers, these are also illegal.
Alicante and Murcia Airport is patrolled continuously by the police & government inspectors looking for illegal touts.
If a Transfer Company asks to meet you anywhere other than in the arrivals terminal or in the free designated parking area for coaches and taxis, they will be illegal and you will not be insured for passenger liability cover.
You should remember that if stopped by the Guardia Civil (Police) the vehicle would be confiscated immediately leaving you stranded. A vehicle that is licensed will be SP (Servicio Publico) plated by the Spanish Authorities.
If you suspect you may have booked with an illegal company or individual ask them to send you a copy by fax or email of their "Targeta de Transporte" license. Legal companies will be more than pleased if you explain the reason for this request.