See which way departures are going when you arrive at the airport, then if possible switch to a seat on the side you want to sightsee from if you're not already on that side.

It's not foolproof, but you could also check the Weather Channel (use the Travel Weather option and put in DCA) or local DCA weather forecasts in the days before your flight - that'll give an indication of which direction the wind is forecast to be from.

Jim