It doesn't really matter whether someone found a leak near the sight glass, there could be more than one leak in the system. If a leak in the coil is the source of the oil that is causing the condensate drip (if oil is the culprit), then cleaning the coil only to have the dripping start back up in two weeks will look to the customer like you're just guessing at the problem. And they'd be correct, because at this point you are only guessing. If OTOH, you find a leak in the coil first, then you can save yourself the trouble of cleaning it, and from the embarassment of cleaning it only to find that it didn't fix anything. HTH.
If there is no leak in the coil, then you're only out a few minutes time.