Try to find out which direction they are going vs your desired direction-
If it is against you then you want to stay out of the current as much as possible- ie tacking close to shore in shallow water vs long tacks out into a channel where the current is typically greater, MUST take into account when you try to judge your tacking/jibing angles. When the current is with you then you want to stay in it as much as possible- ie stay out in the channel and not in the shallow water, still need to adjust your tacking/jibing angles. Prior to the start sail near a bouy/anchored boat/piling and you can get an idea of the current direction and speed. During the race check this as well since if tidal it will change during the day- if river usually doesn't unless close to coast, along beach can also change with location ("rip" currents, etc.).

Hope that helped some-

Kirt