Wot a good question!!
As already mentioned, the upper deck number of windows is not an accurate way to tell the difference.
Also, as already mentioned, the brakes, wheels and tyres are a smaller size, but way to hard to pick from a distance.
So, what's left,
The -100 does not have the wingtip navigation lights mounted at the tip of the leading edge, they are mounted actually about one third chord back along the outboard tip.
The -100 has it's white tail lights mounted at the outboard tips of each horizontal stab, whereas the -200 (and -300) has them mounted in the centre.
That's all I can think of for now!
Cheers