Determine Encryption Used by Examining File Headers?
That depends. A file cannot be directly encrypted by a block cipher. You need at least a mode of operation for that block cipher and an IV. I guess you should first determine the correct encryption format that was used.

If your file is directly encrypted you could try and decrypt using one of the two methods. If the result looks like a file then the right mode must have been chosen. You can e.g. test this by using the file command on Posix systems after decrypting the first kilo byte of the file (into a separate, partial file).

Alternatively you could perform a simple randomness test on the result. If that fails, then it is likely a correct file and the correct algorithm. This may however lead to more false positives than the method used above. You may want to test which of the partially decrypted files is least random and chose that algorithm.