Electronics question concerning LEDs

If I have an Led but don't know its specs, how can I determine the safe voltage (without resistor) for this LED?

I am only aware of the general rules, which go by color:

1.7 V for non-high brightness red
1.9 V for high brightness red
2 V for orange and yellow
2.1 V for green
3.4 V for white and blue

Are there any better methods?

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Electronics question concerning LEDs