I am playing with LEDs and microcontrollers (ESP32). I have a number of pre-wired LED's that I purchased in the past (see picture) but don't know if they are 5V or 12V (I've purchased both), and I'd like to sort them out. How can I do this?

There is no labelling I can find on the LED or the wire to indicate its voltage.

I tried my multimeter in a diode mode, but doesn't work with these LED's (it reads "0L" regardless of of which probe in on which lead).

I don't have a variable current power supply so I can't test them that way.

I (possibly riskily) tried the LED with both 5VDC and 12VDC and it lit up for both.

As a side question, is it true that the LED itself is probably the same for both 5V and 12V and the difference is the size of the inline resistor? Are there any other differences?

I obviously don't really know what I'm doing, and any information, advice, or help is appreciated.