Fox, just follow your instincts.
Well, honestly, I take guesses. But truthfully, it's about reading your opponent's playstyle. Does he take many defensive approaches? Does he take risks? This is why you scout; you scout your opponent's playstyle. If you know your opponent plays WAY too safe, then yeah, Head Smash away (lest he predicts you on that). If he's aggressive, I'd Aqua Tail (but honestly, I don't think it OHKOes Rhyperior, lol, so I'd keep Rhyperior in regardless, unless Aggron uses Magnet Rise, then Rhyp's screwed).
There's always the RMT forums you can go too. I really just pick my favorites - the Pokemon I know I can work with that can work with the chosen Pokemon. Ampharos? Has troubles with Ground-types, Dugtrio on the revenge (not like you can stop that), Encore Clefable (Encorers in general), basically anything faster than it? What to do? Uxie with Grass Knot and Thunder Wave? Why not? Spread the paralyze! This works with SubPerior too.