How to know if a code is Huffman or not without having the probability of each codeword?
If you have a Huffman code, and the codes have lengths $l_i$, then the sum over $2^{-l_i}$ must be equal to 1. In your case, that sum is 1/4 + 1/4 + 1/4 + 1/8 = 7/8 &lt; 1, therefore not a Huffman code. You can replace the code 110 with 11. 

(I am quite sure you can prove that for any prefix code, the sum is ≤ 1. And I'm quite sure you can prove that if the sum is less than 1, then there is some bit that you can remove from some code with the code remaining a prefix code, and since there is a finite number of bits to remove, the sum must become 1 eventually).