They like to say this because it puts the onus on you. Fortunately, very easy to tell. View the modems status page (ip usually on the sticker on the back/bottom) and see what it says. It will tell you how much signal it is receiving, and how much power it has to push to be heard. It may or may not tell you if this amount is "too much" but you can google it and will get a million sources to compare with easily enough. Depends on how many channels you have bonded, but typically anything under 50 is good. 50-55 we'll call yellow, and above 55 you are in danger of damaging your equipment.My problems are always in the upstream direction. As explained to me by multiple technicians, my modem decides it needs too much power to push data up the line so it goes into a continuous reset cycle until magic happens out on the pole and the modem's power drops.
Also, look at those numbers, then plug your modem into the line outside the house (before any splitters) and see if it's magically fixed. Account for the 3db per splitter drop and if your signal is roughly matched (I.E. 51 inside the house, 44 outside and there's two splitters) then the problem is in their network not yours.