Get some soapy water. Pour it over the area where you think the leak is. You'll see a steady stream of bubbles forming where the leak is located. Depending on where it is and how strong it is I most likely wouldn't stress about it.
Leak comes from the hose itself: replace the hose. Use a different reg for the moment.
Leaks from where the hose is attached to the 1st stage: get a wrench and tighten it. If this doesn't fix the problem, replace the oring or get a new hose. Don't dive with the reg until you do.
Leaks from the port plug: get an allen key (hex wrench) and tighten it. Again if this doesn't fix it replace the oring in the port or get new plug. Don't dive with the reg until you do.
Leak from face o-ring (tank and 1st stage meet): replace it or don't, if it wasn't loud enough for you to notice when you put the gear together then it's not worth switching at that moment.
Leak from the neck oring (tank and tank valve meet): worry about it later. After the dive take it for replacement.
Leak from the valve handle (on the tank): don't worry about it (unless it's a LOT of air), and fix it later.
If you're 1st stage has a little black disk on it and the bubbles come out of that disk: as stated in earlier posts it's a function of reg design, no problem.
Because of the way you've phrased the question, the only answer to really give you is to investigate, make an experiential based judgment as to it's likelihood to cause problems while on a dive and fix it if possible. Just like the food in your fridge: when it doubt, dive with a different reg. If you're stressed about your reg malfunctioning (even if the problem is insignificant), you're more likely to make other mistakes or have a less enjoyable dive.
