I believe that I forgot to add the sugar in my sugar cookie dough, but I'm not completely sure. If I did forget, how will the cookies turn out once baked? Will they be edible?
Sugar cookies without the sugar will be somewhat akin to hardtack crackers: they will not be sweet at all, and without the moisture retaining and softening affect of the sugar, they will be much harder.
They will certainly be edible, but probably quite nasty.
If you are not averse to risk from the raw eggs, taste your dough. It should be apparent whether there is sugar in it or not.
If you omitted it, you can try to mix some in now, but this will also continue to work the flour, creating more gluten, and thus giving you a tougher final cookie; therefore, you may choose simply to discard the batch and start again.