Find Voltage between two terminals
Not many "tricks" for this one. Best to choose a reference point from where you measure all the voltages. By habit, most of us would choose the bottom-most node (I suspect that @Transistor has chosen this point when he asks "What the voltage at 'a'?).

One trick for this particular circuit is to recognize the symmetry. One branch is the inverse of the other branch. So you need only solve either 'Va' or solve 'Vb' voltage. Then compare with half the supply voltage. Double the difference to get the solution of 'V(ab)'. This 'trick' may seem a bit round-about.

A different way of viewing the symmetry trick takes the voltage 'Va', and applies Kirchhoff's voltage law: 'Va' + V(ba) + 'Va' = 12V. Be wary of the signs of these voltages...is the required solution "voltage on A with-respect-to-B", or is it "voltage on B with-respect-to-A" ?. It may be best to state your solution: voltage on B is more positive than voltage on A by X volts...this is unambiguous. As a sanity-check, if the magnitude of your solution of V(ab) is larger than 12V, you've made a sign-error.

Are tricks worth the trouble? A non-symmetrical choice of resistors forces the long solution: solve voltage 'a' &amp; 'b' in each branch separately, then subtract to find the difference. This smells somewhat like superposition, where signs of voltages or currents can drive you crazy.