Rehabilitation of cheaters.

BLUR, let me put it this way - if you can't figure out how to check whether your opponent is cheating, then just don't think about it.
Focus on your own game, build your town, grow your army, and stick to your life goals. And in Heroes, assume that your opponents are not cheating.
If you keep learning how to play Heroes, the answers will come to you.
There is no definitive answer to your question - you need to pay close attention and think critically in each specific situation.

P.S. If you are a cheater and were just testing the waters, then the answer is this: in your example, it's impossible to catch them. And in real life, the optimal solution would be to rob a bank.

Feeling something is one thing, but actually being able to prove that someone is cheating is quite another;
I'm playing right now, and it seems like every other player is a cheater. What's the point of continuing to play with them? I have enough experience in this game, although not at a professional level. Sometimes I wonder if I'm missing something. And indeed, I only recently learned that the hero's movement speed depends on the speed of the creature with the lowest movement speed. I didn't know that, but I suspected it, as I've been playing Heroes 4 for a long time, where this is more obvious. And recently, I actually had to replay a game from my opponent's perspective (because I was tired of these suspicions), and I also had to check all possible hero moves several times to understand that he had cheated. Why do I need this hassle every time? I can't understand: why cheat in Heroes 3? Is it just to satisfy your ego? What's the fun in the game?