There is a function on the Player class that returns the difficulty setting for that player. (Player.Difficulty() I think). It should be a value from 0-2 if I remember correctly. You can just use this to determine which difficulty setting to select internally. Usually, there is a switch statement, or a few if statements that setup initial resources based on the difficulty. Then there might also be extra disaster triggers that get setup for harder difficulty settings.
Something along the lines of:
switch(Player.Difficulty())
{
case 0: // Hard or easy? I can't remember
// Put code for this difficulty setting here
break; // We don't wan't to continue on into the next case
case 1: // Medium
// Code for next difficulty settings goes here
break; // (Hmm, I almost forgot this one)
case 2: // Easy or Hard? Note: You can also use "default:" here instead of "case 2:"
// Code for final difficulty setting goes here
break; // This one isn't really needed, but why not? it's probably safer to have it here just in case you edit code and forget about it.
}