A few thoughts and comments. First and foremost, with true randomness, variability is expected, swings happen. On the days you happened to be tending lower in a batch of merely 100 rolls or so, someone somewhere may have been tending higher for dozens of rolls, and other people were getting a wide range without a specific swing or trend, but overall across everyone, the distribution is the expected curve (across millions of rolls). So you've just happened to have some small batches of bad rolls out of the millions. It's one way of looking at it. But to answer your question. (EDIT: Mirrors what al said). If your players are rolling In-Line Macros (where the dice result shows up in a little yellow box instead of the whole dice formula showing), then the DM can mouse-over the yellow box to see what kind of math they're doing, and make sure they haven't hidden some trick into the formula. That's about the only way I can think of to do a chatroom-trick that could be called a dice cheat. Note that if they are rolling off the character sheet and it looks like a complicated rolling formula that doesn't mean cheating -- many of the Character Sheets generate complicated rolling formulas that appear when you mouse-over the inline roll. The cheat/trick way is like if you roll a d20 and put the code to reroll any 1's, 2's, 3's, 4's, and so on, if you're just claiming to roll a plain unmodified d20.
