Grain of wheat, rice, whatever, are filament bulbs, and for making things that sit there and glow, do take a lot of current. They do have the advantage that it doesn't matter which way round they are connected.
LEDs are diodes that have the magic property of glowing a very pure colour at a low current, but they dont liight up their surroundings like a bulb. Unless you get the super high output ones that do burn almost as much juice as an equivalent bulb.
The safe way to check an installed one when you dont know which wire is which is to use a knackered* 3volt dry battery (e.g. 2 elderly aaa's) as a test supply, and see which way round gives a glow. The voltage is less than the max reverse, so no damage there, and the poor current delivery will not bust them in the forward direction.
*technical expression