1. remove it from the circuit - other stuff can cause erroneous readings. Unsoldering any two of the three terminals is OK.
2. Use your ohmmeter to measure the resistance from outside lug to outside lug. Write this down. This is the potentiomenter's resistance value.
3. With the shaft pointing at you, lugs down, turn the shaft all the way counterclockwise. Note the position, perhaps by making a mark on the end of the pot shaft. Now rotate the shaft fully clockwise, and note where the mark has turned to. This tells you how much mechanical rotation the pot has. Carefully estimate where in the mechanical rotation the halfway position - equally distant from the limits of rotation either way.
4. Clip your ohmmeter onto the center lug and the left most lug, and rotate the shaft fully counterclockwise. The resistance should be very low, typically only a few ohms. Now rotate the shaft until your marker is at the halfway rotation point. Note this resistance.
5. Figure out your pot's taper. A linear pot will have a resistance that's proportional to the rotation - that is, turn it halfway, and the resistance will be about half the total resistance. An audio or log/logarithmic taper pot will have changed about 10% of the total resistance at the halfway point. A reverse log or reverse audio taper will be about 90% of its total resistance at the halfway point.
And now you know!!