I have an expensive accurate Fluke multimeter. It can show the forward voltage of ordinary diodes and red, some green, orange and yellow LEDs but not white, blue and some green LEDs that have a higher voltage.
I found some bags of LED Christmas lights thrown away that had some LEDs with their wires rusted away. I tested and harvested hundreds of them. Most in a string have almost the same forward voltages and therefore were connected in parallel. That is how I am wiring them in my new LED projects.