The leak-dye they installed when they serviced the system will show up using 'black-light' and the inspection of the system should be included in the price you paid [ I assume the system was empty and they tested with an electronic detector and couldn't find the leak then....sometimes I assume too much]. This recheck only works on visible engine room components but is very effective for slow leaks, when you can see them.
The most common 'unseeable' leak is the 'Evaporator' inside the heater box and those are easily tested with a 'sniffer'. replacement requires removal of the glovebox, cover panels, firewall cowl cover for pipe access and removal, but it's not a difficult repair and if your quoted more than 5 hours labor....shop elsewhere...........jb