with a heavy tactical rifle in .308, the chances are slim. One easy trick is this, be sure there is no scope shadow. If there is no shadow in your site picture, you know you are in the optimal eye relief range. Of course, cheek weld is critical for accurate shooting, it has to be the same no matter what. So, if all the pieces are coming together, same cheek weld everytime, which will be where you have perfect scope picture, which is where you have perfect eye relief, etc, then you should be fine.
Another thing to do, is when mounting your scope, get in your firing position and move the scope as far forward as you can, then move it back until you get complete scope picture, and thats where you mount it at. This means you will be at the BACK of the optimal eye relief range. (it usually covers .5 - 1" of distance (i.e. 3.5-4", etc. Read your scope manual to find out what it is if you are interested). Also, if you ware doing this with a variable power scope, do it at the highest magnification, which usually has the shortest eye relief range. Otherwise, you wont get complete scope picture at high mag.
MEL