If the cheater isn't an idiot, but a qualified chess player using a computer in moderation, nothing can be detected. Of course, a candidate master crushing Carlsens and Kramniks won't fly, but careful adjustment of the result is quite realistic. This way he's one of the favorites, and with a computer, he's guaranteed to take first prize (if playing honestly wouldn't work). Or otherwise he's just filler, but with a computer, he shares 3-9 place and gets some prize money. The main thing is to play practically at your own strength and use the help of an iron friend only at the necessary moments, and no one will prove anything.
That's why now online they mostly play blitz, where a serious deterrent is the time to enter moves into another computer (otherwise the program will catch it) and waiting for the computer's decision.
At the same time, even in over-the-board chess, scandals periodically arise (a sudden increase in playing level, "getting lost" in blitz, at secondary competitions and various "fun games", frequent and short trips to the toilet, moves inexplicable by human logic, gross errors explainable only by a "broken telephone", sharp changes in strength of play, sometimes several times during one game, etc.) But even then, proving the fact of cheating based on such circumstantial evidence is very difficult, and a sensible chess player wouldn't allow such gross blunders in the first place.
Even more so in Heroes; if you can somehow open a save, a clever maphack is unprovable. Not everyone is an idiot to run, like Dsquared, into the darkness for utopoy and save on scouting.