In datasheet on page 30 described "Measuring LVDS Input Timing Skew".
For example, to compare DB15P/N to DCKIP/N, first write 0x60 to register 0x18 to set LMX_EN = 1, LMX_ADR = 10000, and LMX_SEL = 0. The signal from DB15P/N will be driven onto TSTP/N. Write 0x61 to register 0x18 to set LMX_SEL = 1 and cause DCKIP/N to appear at TSTP/N with inverted polarity.
I don't understand mechanism of measuring it. When I switch to DCI how I will know the skew between signals because for once I can see only one signal.
You look at the phase of one relative to the other, this will give you the skew.
If you are looking at it with a scope you can save one waveform and compare it to the adjacent waveform. You can start with one side of the data bus and compare output signals until you get to the end.
Below the example of acquired DATA and DCI. How from it possible measure skew?