All taxis that enter the airport area at LIM are checked for documents (license, insurance,etc.) and are not permitted to enter without.
In addition,they are subject to penalties should they get busted without docs.Hence, if they are in the airport, they are legit.

Experienced travelers with a little Spanish and haggling skills can score cheap rides out of the airport by going directly to the drop off area straight out front, and getting a cab that has just dropped off a fare.

Check prices inside the terminal and cut them in half to start.

Speed and discretion will work to your advantage, so as not to rile up the taxis waiting in line.