Several - obviously a lot of it is common sense. But, players tend to have signatures in their tactics as well. For example, imagine the following (a bit of a silly example)
Turkey: A Smy, A Ank, F BLA, F Syr
Russia: F Sev, A Arm, A Rum, and no possibility of Rum moving anyplace successfully.
Turkey wishes to destroy Arm using the army in Smy. There are two sets of orders that will accomplish this. Different players will tend to issue different ones of these sets, but do so consistently, and there are a lot of thematically similar decisions that the same person will tend to make in the same way.
There are some more complicated examples, too. Basically, whenever playing a gunboat game, it's a good idea to start by analyzing the game from the point of view of everybody else, and attempt to predict what moves they're likely to make. If you have 15 game years to analyze (that's 100-200 moves for the powers involved), you'll find that there are some powers you're consistently getting right and some you're consistently getting wrong, for this same sort of reason. What you'll often find from this exercise is that two close allies played by the same person will be equally predictable/unpredictable, but two close allies played by different people won't be.
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