If it's not freezing, then it's not too cold to crack the lid of the hive for a brief peek. If your bees are occupying 80% of the frames then add another box, otherwise leave them. They will have an easier time of keeping things warm without the second storey. Once it warms up you can do a more detailed analysis. A frame of capped brood will hatch out and cover two frames within a week on average. A frame of uncapped brood will hatch out in about two weeks and cover two frames. So you can anticipate the needs of your colony quite accurately.