Hi Summer - sorry to hear your tale but it is quite common here in Spain.
Really you will have no idea what was or wasnt charged to the tenant but you need to get hold of the contract as if the tenants are spanish then you could be in for a huge problem. The spanish tend to assume a vivienda contract which means a 5 year term of permenant living.
Your agent should have a written agreement from you to allow them to handle the rental so you may have a comeback with them on that ie here in Spain in order to enable to do anything for anyone, strictly speaking one needs to issue a power of attorney. With the power then they can issue a contract between you and the tenant - without it in theory you can say you havent rented the apartment BUT you would need to go to court to gain possession.
There is an arbitration system for rentals but it is very overloaded and takes time.
There may have been a notice to quit at the signing of the contract so without that document you have no idea what is going on
Also if the rental agent took cash and therefore didnt declare it, again you may be able to hold their attention as it were and they should help you
Try and get hold of the contract and then get this to a lawyer to find out what your rights are and hopefully you can do something.
if there was no written and signed agreement/contract and the tenant has only paid a months rent, then effectively it is a rolling month on month agreement and you can ask them to leave - give notice in writing.
All owners should ensure they know what their agents are doing and SHOULD sign an agreement with them so everyone is in the clear as to what is going on - it protects both parties against any arguments!