I love playing some of the old Avalon Hill games like PanzerBlitz and Midway (1964 edition) (I have the 1991 edition as well, but haven't played it much).

In PanzerBlitz, there are generally two approaches that work well: the widely-spaced unit approach and the heavy stacks approach. However, picking which method to use seems highly biased on how the other player will operate (ie, you've played against them several times and have gained an understanding of their tactical methods).

Likewise in Midway, there are only a couple viable options for the Japanese player - come on full strength, or spread way out and converge from multiple angles from their entry side.

Many other games follow in these two examples' footsteps: what is the best way to gain an understanding of a particular opponent before playing a game with them? Or, asked another way, how can you quickly change your playing methodology based on your opponent?