Suppose I'm given an operation $\displaystyle *$ on a set $\displaystyle A\,\, \text{where}\,\, a,b \in A$ such that the operation between two elements of set $\displaystyle A$ is also an element of set $\displaystyle A$

Now there are $\displaystyle 2 \times 2 \times 2 \times 2 = 16$ possible binary operations of $\displaystyle *$ on set $\displaystyle A$.

If I'm asked to find if there exists a neutral/identity element of a certain table where $\displaystyle a * a, a * b, b * a \text{ and } b * b $

are defined, how do I figure that out?

What I mean is suppose 1 of 16 table of operations looks like this:

$\displaystyle \begin{array}{|l||r|} \hline (a * a) & a \\ \hline (a * b) & b \\ \hline (b * a) & a \\ \hline (b * b) & b \\ \hline \end{array} $

How do I figure out if an identity element exist for this table? Also is it possible to find the identity/neutral element from this table?