ASI (Adjacent Satellite Interference)

Use a span covering the whole band. Video averaging ON. Use MAX HOLD if looking for an intermittent interferer such as a burst from a TDMA site.

Use narrower span, as well, if looking for specific interferer. It generally helps to see the whole picture to start with.

Point the dish at each nearby satellite and record the spectrum on both polarisations.

Correlate with your wanted spectrum, paying particular attention to the gaps between carriers on your wanted satellite.

Turn off your own carriers to see the noise floor under your carriers.

Some interference will be due to your own sidelobes.

Some interference will be from sidelobes of uplink sites towards the adjacent satellites.  Make sure you understand the uplink frequencies and transponder cross strap configuration of your own and adjacent satellites. If you see interference from earth stations uplinking to adjacent satellites contact the  the NOC for the relevant satellite.

Checking on a CW carrier gives very detailed information and you can detect and measure signals from many satellites, even very far away. An external PLL type LNB and atomic standard 10 MHz reference helps.

On a budget, consider using a FUNcube dongle https://www.funcubedongle.com/ and SDR-radio software https://sdr-radio.com/ ; I would be interested to hear how you get on.