daft question - what does 'bagged up' look like? And how soon to popping is it usually judged to be?
If you look at their udder it will be more developed than usual, and the teats will be obvious. I tend to crouch down and have a look when the girls are eating. If they have a lot of wool you might need to part it in front of their rear leg to get a better view.
Mine also start to walk like John Wayne at the rear

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How soon - could be weeks, could be hours

. The older ewes, from what I've seen, bag up earlier. Mine tend to bag up the same number of days before lambing every year, so if you make a note this year and then you will know for next.