all LEDs work on approx 2-4 V (depends on color) and you should put some resistors for voltage drop - to stabilize circuit (limit the current) so voltage should be higher. LED is semiconductor, not resistor like classic bulb. I found you nice text about LED voltage and how to create circuit. See the link down.
However, most of diodes will work ok if you put them on 3V battery and if you put them direct on higher than 6, they will die. if you put them parallel, you need resistor for each of them, voltage is same, but current is higher - if you have 12V supply (car battery), you can put them serial with one resistor (make calculation to design circuit) so it's cheaper and more simple - but if one is not working, all circuit is stopped
You can't measure the power supply - you need to calculate what you need by knowing your power supply voltage. Typical current for diodes is 20 mA so with that info you calculate which resistor do you need.
LED Basics; gaining an understanding of how to work with 3 and 5mm LEDs
here is wiki if you want to learn more about structure of diodes:
https://en.wikipedia.org/wiki/Light-emitting_diode