Two sine waves of the same frequency go to the X and Y inputs of a scope. The trace is a straight line that slopes up to the right. What is the phase shift?

Correct answer: 0 degrees

A straight line means the two signals rise and fall together, so they are in phase. A shift of 180 degrees would also give a line, but it would slope down to the right.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 12.2: Frequency and Phase Measurement