Count the LED's (in series) and multiply by ~2.8V
I did this the other day, get a variable power supply that can go up to 50V and has a current limit.
Plug in the LED's to the supply with the correct polarity (and make sure the electronics are removed)
Then set a current limit at say 50mA and set the voltage high, like 40 or 50V. Then keep turning up the current limit and watch the voltage go up. When the LED's get about as bright as they are in the bulb then record the voltage and that's how much they would take.
Make sure you don't apply too much voltage, because it could burn them out.