I posted a topic here recently, asking about how to know what resistor to use for different applications. Someone told me that in the data sheet, you can find the forward voltage of a component. Well, I don't have the data sheets. Could someone please give me the link to the most generic and basic ones?
Thanks in advance,
~f(x) = (18x + 5x - 23x)2 f(4)
That will hardly help you, you still might blow your LED by assuming it is generic.
If you have a multimeter with diode (or rather continuity) test, you can find out that forward voltage.
My multimeter has such continuity test when set to the symbol in the attached picture.
Hold the red wire to the longer LED leg (anode), and the black to the shorter leg (cathode).
The value shown would be your forward voltage (if within range).
But you also need the current in order to know what resistor is needed.
If you don't have a multimeter, can you tell what color those LEDs are ?
Here is a discussion you may find helpful. Overview | All About LEDs | Adafruit Learning System
See colors and materials. Light-emitting diode - Wikipedia
ALL single LEDs have a voltage drop within 1+ to 3,5 V depending of colour and size.
ALL LEDs can cope with 10mA (some MUCH more)
so as a safe rule: use 100 * (Voltage-1) for a series resistor.
Working in the 5V arduino world a handful of 220-330 ohm is handy
(even up to 1k will give you a light - and reduce power consumption)
MAS3:
If you don't have a multimeter, can you tell what color those LEDs are ?
I don't have a multimeter, but I have red, green, yellow, blue, white (I guess, I haven't used it) and one with four leads that I don't know anything about 
~f(x) = (18x + 5x - 23x)2 f(4)
Red is usually the lowest, ~2.5V.
So (5V - 2.5)/.02 = 125 ohm
You can also use a 1K with each color and measure the voltage across the LED.