With A and B, it's pretty simple. If you rotate the molecule about its principle axis to where the orbitals line up, and the signs change, it's Mulliken label has a B. otherwise, it's A.
I heard E symmetry means there are two configurations that are equal in energy, and T means there are three configurations that are equal in energy. My professor posted some slides that included examples of E symmetry and T symmetry, but it's still confusing:
Why is this E symmetry? It shows a configuration with one sign along the Z axis and the other sign for the other two axes, and then a configuration with one sign along the Y axis and the other sign for the other two axes. Why can't we have a third configuration, with one sign along the X axis and the other sign for the other two axes, making this have T symmetry? Could somebody explain what E and T symmetry are in their words?
EDIT: I just talked to my professor, and I forgot his exact explanation, but he said the above diagram has a mistake in it. He said two of the orbitals should not be there.