Q:

Checking user input?

i want to check whether the user input is a number if it is continues the code if its not it re-asks until they enter a number 
# This progam will simulate a dice with 4, 6 or 12 sides.

import random

def RollTheDice():

    print("Roll The Dice")
    print()

    ValidNumbers = [4,6,12]

    Repeat = True

    while Repeat == True:
        Counter = 0
        NumberOfSides = input("Please select a dice with 4, 6 or 12 sides")

        if not type(NumberOfSides) == int or not int(NumberOfSides) == ValidNumbers:
            print("You have entered an incorrect value")
            NumberOfSides = int(input("Please select a dice with 4, 6 or 12 sides"))

        else:
            print()
            UserScore = random.randint(1,NumberOfSides)
            print("{0} sided dice thrown, score {1}".format (NumberOfSides,UserScore))

            RollAgain = input("Do you want to roll the dice again? ")

            if RollAgain == "No" or RollAgain == "no":
                print("Have a nice day")
                Repeat = False

            else:
                NumberOfSides = int(input("Please select a dice with 4, 6 or 12 sides: "))

A:

First off, general programming practice: use camelCase or under_scores for variable names.
Here's what (I think) you want:
validNumbers = [4, 6, 12]
repeat = True
while repeat:
    userNum = input("Enter a number ") # input returns a string
    try: # try to convert it to an integer
        userNum = int(userNum)
    except ValueError:
        pass
    if userNum in validNumbers:
        repeat = False
# do everything else

