Hydrometer use is key in brewing anything, so I suggest picking one up asap, they're only about 8 bucks. That being said, the only way to know if your stuff is done is to take a hydro reading. Airlock activity should never be used as an indicator of fermentation, that can lead to some dangerous bottle grenades once you bottle.
I've used welches concentrates before, and never had a problem, so you're probably fine there. The amount of sugar you had in there to begin with was probably fairly high, but montrachet yeast can cut through it fast, particularly with the nutrient you added, and if it was at room temp. If it looks fairly clear, and there's a good compacted layer of sediment on the bottom, I would rack this to a secondary and take a gravity reading. When you gravity hasn't changed for about a week, then you're done fermenting.