Photoresistor LDR in parallel
At first glance you might think that with 50 sensors dark and 100 k&Omega; each, you get 2 k&Omega; with all in parallel.  With 49 sensors at 100 k&Omega; and one sensor at 10 k&Omega;, you get 1.7 k&Omega;.  Going from 2.0 k&Omega; to 1.7 k&Omega; is a 15% change.  You really should be able to detect that.

However, it's not that simple.  CdS LDRs, like you're almost certainly using, are notorious for having wide tolerance.  They won't all be nice and neatly 100 k&Omega; when dark.  That value will vary considerably from part to part, with temperature, and with time.  The lit resistance will vary too.  In the end, I doubt you can find a overall parallel resistance threshold for reliably detecting at least one sensor lit.

If you were really forced to put all the sensors in parallel, then you probably need to calibrate to dark before each use.

Instead of just connecting all the sensors in parallel, I'd put a little active circuitry at each sensor.  This could be as simple as one additional resistor and transistor.  The transistor turns on when its cell senses light.  It then pulls down on a common open collector line.  This scheme makes use of the much higher signal range at each sensor, as apposed to all sensors averaged with one of them changing.  The resulting digital signals are then ORed.

This does require three connections (power, ground, and output).  You don't always get something just because you wrote a spec for it.