Q:

High voltage LED indicator 40-1000 VDC without additional power supply

I would like to make an indicator to show that DC bus (700VDC) capacitors are charged (be careful!).
What is the best way to make a LED indicator, which will work for a long time from 40VDC to 1000VDC without additional power supply and with minimum power losses?

A:

Connect a moving coil analog voltmeter across the power bus. 
Either a voltmeter as shown with internal series resistor or an external resistor
and scale calibrated for the desired range. Photo from this useless site. 
Old-school suppliers such as Crompton should be able to supply a meter with the markings you need, if not a turnkey solution. 

A:

Something else to consider is to build a relaxation oscillator using a DIAC, capacitor, LED, and couple of resistors.

DIACs are still readily available, although Digi-Key wants to sell them in full reels. They can be found at most electronic suppliers as well as places like eBay, AliExpress, Banggood, and DealeXtreme.
The advantage of using a relaxation oscillator rather than driving the LED with a large-value resistor is that the LED remains visible (flashing) with low voltages applied. It will stop flashing when the input voltage drops below the sum of the DIAC trigger voltage and the LED forward voltage:

A:

An example of a reliable economical no-power-supply (less than US$10) indicator solution that does not cost more than US$200, like the other meters. :(
Since this coil draws 50 µA full scale it is equivalent to 10 V/50 µA= 200 kohm. And thus at 1 kV the R load is 50 mW full scale with 1 kV/50 µA = 20 Mohm 1% or +/-200 kohm.

simulate this circuit – Schematic created using CircuitLab
It also draws the least current and is readily available.

