Is there any method by which I can tell whether a given alphametic is valid or not?
To validate a given alphametic, that is, confirm if it has a unique solution, the easiest way is to submit it to an alphametic solver (computer program).
In the Web you find four major online solvers:
If you test the puzzle TWO + MEN = MEET you will find that it is not a valid alphametic, because it has no solution.
If you test the puzzle BILL + GATES = GREAT you will find that it is not a valid alphametic, because it has 74 solutions.
But you can try testing PLACE + A + CALL = LATER and ... bingo! you find that this is a valid alphametic, which has got a unique solution: (67142 + 1 + 4177 = 71320).