How can I determine whether I have moisture issues behind a brick facade
Seems like there are a couple of things going on here:

First, you say, “next to the window.” Typically, I see a lot of leak problems around windows and particularly at the corner of the sills. This is a “leak” issue and not a lack of moisture barrier issue, as implied by your title question. 

Yes, darkened wall sheathing is an indication that moisture has been there, but has dried out. If the sheathing is solid (not punky) and stained on the surface only not throughout, then it would indicate that not much moisture has entered or not very frequently. 

Second, you don’t mention a moisture barrier between the wall sheathing and the brick, but if there isn’t one, then You have a bigger problem. Hand-in-hand with the moisture barrier is the need for “weep holes”.  I’d check to see if there are weep holes and if they’re “open”. 

Where I live, brick veneer is installed with a 3/4” to 1” air space to the wall sheathing with a moisture barrier on the face of the sheathing. The brick veneer extends down about 4” below the top of the floor line (finish floor) with weep holes at the bottom to allow water that gets behind the brick to drain out. 

Third, brick is porous and needs to be sealed with a clear liquid sealer from time to time. Wind driven rain will “push” the rain water through the brick veneer. That is why a moisture barrier and weep holes are required. 

I’d verify if the “leak” problem is at the window only or lack of moisture barrier and weep holes. If the darkened sheathing is punky or darkened throughout the sheathing, then it could have mold and you may want to have it tested. 

If the brick wall is 8” thick and not 4” thick, then it’s a structural wall and not a veneer wall. We usually don’t use a moisture barrier for structural walls.