What does "strong enough to winter" actually look like — is there a frame count of bees I should be seeing?

Ontario, second year, three colonies. Everyone tells me the most important factor in winter survival is going in strong, and nobody will tell me what strong means in a way I can check. My three colonies are clearly different sizes, but I have no idea whether I'm looking at two good ones and a marginal one or three fine ones and me worrying. What I can measure without a real inspection. Number of frames with bees visible between them when I crack the lid on a cool morning — hive A has bees in eight gaps, hive B in six, hive C in four. Weight by hefting — A and B are heavy, C is middling. Bees at the entrance on a mild afternoon — A lots, B plenty, C noticeably fewer. Is a four-gap colony in October viable in southern Ontario, or is that the one I should be uniting to hive B this week while it's still 12°C? And is the frame count even the right measure — I've read that the quality of the bees matters more than the number, meaning young well-fed winter bees rather than worn-out summer ones, which I have no way of assessing at all.