If nothing is obvious, including any lines for draining or backwashing, try to find out if it leaks more with the pump on, or off. Measure water loss with the pump on for a day or 2. Then turn it off for the same amount of time and measure the loss. If it loses more with the pump on, it is most likely to be in a pressure line. More loss with the pump off indicates a suction line. No difference indicates the pool itself including around fittings, lights or even a hydrostatic relief valve. Grab some test dye from your local store. With the pump off and the water still, slowly squeeze a bit of dye around suspect areas to see if the dye tends to be drawn out of the pool. You need to keep everything very still though unless you have a large leak. The water leaking out from the pool will draw some dye with it and you have found a leak. Hopefully the only one

The above at least gets you going and will save a bit of time (hopefully) when you call someone in to deal with it.