Pete-
Another way to try to figure out which way it is running is if you can take a "transit"- ie sight between two stationary objects on one tack vs the other- all boats "sideslip" a bit but it should be noticeably worse on one tack vs the other- whichever it is worse on is the direction the current is moving- of course this only works for "cross currents". If you have a compass aboard you can use it as well to take a bearing and get the same information.

Kirt