So, in both cases, the visual appearance of the test signal is a yellow-green colored screen of varied intensity from dark on the left to light on the right. If you don't have either test signal available, then I'll suggest the Videotek VSG-21 sync and test signal generator. That unit makes four different test signals plus Black Burst and Audio Tone. One of the test signals is a modulated stairstep signal and it is appropriate for both diff gain and phase measurement. Differential Phase Diff phase is shifting of variance in the phase of the modulated stairstep or modulated ramp test signal over the period of one line.
Measurement is made with a vectorscope and is viewed as follows.
This second dot is displaying the burst saturation and phase as well as the modulated chroma saturation and phase. And, if all is well, both are at the exact same place so you will see only one dot. Diff phase appears as a smear in either clockwise or counter clockwise direction from the center of the color dot. Any variation in the center dot are problems in black and white, and are not addressed in this discussion. Measurement of diff phase is then accomplished by direct measurement of total smear in both clockwise and counter directions.