You can test that by pulling the fabric in different directions. The straight of grain has the least amount of give, the cross grain has a bit of give and the bias has the most give.
Also most garment pieces are patterned so that the straight of grain runs straight up and down. This allows the fabric to "drape" gracefully along the body.
So if you can't determine the exact grain, start with assuming that it's straight down the center of the pattern piece and do a mock up. When it's in fabric you'll see right away if the grain is wrong because you'll get a lot of what we call "Drag Lines" or the wrinkles that let you know where something is off.