I think the most important thing to use is a clock. That is, record the times when the interference starts (I know, hard to do at times) and when it ends and even the way it behaves with starts and stops over a period of time. Unless the interference is due to something continuous going on inside the ham's equipment, the interference should not be continuous but have breaks (starts and stops) due to the nature of ham radio communication. Hams still keep log books (usually computer based) so this date and time information is the best evidence for identifying the source.
And, the second most important thing is a recording of the interference itself. After the ham operator is convinced it is him causing the interference (due to the information of dates and times) he will want to know what it sounds like to give him hints to the problems. It could just be that high power is overwhelming your very poorly engineered electronic equipment. Take no offense, almost all commercial electronic equipment found in homes today is poorly engineered to protect from RFI.
When some neighbor approaches me about interference the first thing I ask is when -- that is, the date and time of the interference. Usually the answer is vague (like, "this morning") but with one particular neighbor, even the vague answer was sufficient for me to claim that I was not operating. I actually showed her the detail of my log book to demonstrate my proof that I was not operating at the time.
It turned out that this neighbor saw my antenna on my roof so any problems or unusual activity of her TV set (her complaint) were blamed on me.
Another neighbor complained of my CW signal coming through his stereo system speakers. Now, I believed this complaint right off because he could almost copy the signal himself being a former (years ago) radio operator in the Navy. It was only occurring when my power was above 1000 watts and since he was next door likely a near-field interference. I put a number of ferrite RF chokes on his speaker cables and that solved the problem.
By the way, today's modern TV sets are fairly immune to RF signals unless they are really close to the signal's near field. For example, at about the 700 watt level with CW, I can often turn my TV set on (well, I have since fixed that problem). The problem was the attached DVD player. The signal came over the DVD player power line plug and then turned on the DVD player. With my configuration, when the DVD player turns on, the HD TV set automatically turns on and switches to that HDMI port even if there is no DVD in the player. Fixed again by using about a dozen small wraparound style ferrite RF chokes on the power cable to the DVD player.
If the interference is not due to a local ham radio operator, some other likely sources could be commercial equipment known to be RFI sources like: Plasma TV sets (some vendor's), Fish Tank heating regulators, Wall-wart style switching type power supplies, Exercise treadmill!! [Also a light dimmer. These are notorious for wiping out a section of a band(s)] My neighbor two doors down from my house has an exercise treadmill that can put a 40/9 signal on my receiver's S-meter.