You could just search for 'comparator' on google.
You set a 'reference voltage' on one pin....then the voltage you wish to compare it to on the other, the output will change state when the input goes above/below the reference.
Obviously, a comparator like the lm393 (which has 'two' comparators in it) cannot take 240v, so a suitable voltage divider would do..to get it down to, say, 10v. Then set your reference at 10V and bobs your aunties brother
I'm not sure of the VVC range, but I'm sure it'll go from 12v.
Maybe not as simple as a 'transistor', but comparators are cheap.
Failing that, a (not very accurate) method would use CMOS logic, as these have a set transision voltage for a 0 or a one...usually VCC/2. That is, an inverter (4069?) will output a 'low' when its input goes above 2.5v. So for 240v you'll need a voltage divider of about 96. (470k, 4.7k?)
The 'flash indicator' on a camera is a simple neon lamp, usually connected across the charging cap via a voltage divider (and current limiting resistor) resistor. It lights at about 90v (ish) so if the voltage divider has a ratio of 3:1, it wll light up when the cap reaches 270v charge. Simple, but relatively effective.
The comparator idea would be far more accurate, but if your caps are rated at 120v...and you charge two in series...I wouldn't go for 240v...thats pushing them at their limit