If you think your opponent is fighting dirty then you can review the entire game after it is over. Perhaps he was cheating, but this is almost never the case. Much more likely, you will learn a perfectly good strategy that you can use on your next game. In this case, "fighting dirty" is more akin to "geurilla warfare" in that it isn't a cheat or exploit, so much as a high-damage, low-cost attack reliant on either Stealth or simply a mind game of what your opponent is expecting, and what he isn't.
Some "dirty" tactics include, but are not limited to: using production experimental units like Fatboys and Tempests to build an army in a spot your opponent wouldn't expect, capturing secluded outposts and using his own units against him. While usually thought up on the fly, some maps have areas that are seemingly purpose-built for these tactics, such as the tiny islands on the outskirts of Seton's Clutch, which make a great staging point for an unexpected assault or a strategic missile launcher that can strike from an angle your opponent has no SMD coverage against. There are hundreds of "dirty" tactics, all of which are fun and effective if executed properly.
The best counter to all of these "tricks" is good intel, which you should have anyway. Tricks like these also highlight why T1/T2/T3 Spy Planes remain useful into the very late game, because spots like the Seton's Clutch islands are usually out of range of Omni. Maintaining complete battlefield awareness can stop virtually all of these attacks before they happen.
The phrase 'fighting dirty' is really just a matter of opinion. Some players think Experimental Units or Strategic Missiles are dirty, but these units are implemented in the game and closely balanced. There are, however, special rules which can be activated before the start of a game which prohibit the building of any number of units or buildings.