Is there an easy way to do so?
Connect to constant current source (e.g. lab PSU), measure voltage drop across the LED.
And if I dont have a PSU? I've read about connecting a 1k resistor in series..
Got a hobby-charger? A hobby-charger in NiXX charging mode is pretty much a CC PSU.
Vf varies with both current and temperature (-3 mV/°C for XM-L).
Depending on what the purpose of measuring Vf is, you might need to use the full drive current and also mount the led on a heatsink.
To just find the led with lowest Vf, you can probably get away with a lower current and then do a fast measurement, before the led gets warm.
So I can just light the LED up and measure the voltage at the LED + and - tabs and that will be the vF?
Yes, but only for the actual current, another current will give another Vf.
Can the vFs at 2A vary by a huge margin if I measure the same vF at 350mA?
A bit above 0.4 volt.
Check the datasheet, it has a curve with typical Vf (Forward voltage) at different currents.
Edit:
I think I misunderstood the question. If two leds has the same Vf at 350mA, they will probably also have nearly the same Vf at 2A.
Yep, your edit nailed it. :)
Thank you very much for your help.
Easiest way to compare is ubiquitous 7135 driver with full battery and just put DMM across the LED.