If not, how much can they carry and how can you tell where their limit is

They do have a limit, items will gray out when it is too heavy. Eg: You get near the limit and an 8 weight item is grayed out, while a 4 weight is still white/lighter gray and can be passed over.

Not sure the exact limit. Took someone with me and guesstimated she was able to haul ~250 or so back. Probably varies between companions.