Eureka! As Archimedes said, according to legend.
In principle, "TheMachineCharmer's" answer is feasible, but I would recommend recording the change in the volume of water instead (if you need an accurate measurement), because (1) it could be difficult to measure the volume of the spilled water, and (2) it is also a little less accurate to do so. (Some water will be left on the sides of the first container, and inserting the object into the filled container, catching all of the spillover while making sure the water level doesn't drop below the brim, could be difficult.)
If you only need a rough idea, the other way is fine.
If you need a more accurate measurement, try one of these:
Get the volume of a container by filling it with water (e.g. from a graduated vessel). Empty the container, then place the object in it. Fill it with water again, measuring how much water you added. Subtract this number from the volume of the container to find the volume of your object. (The order in which you do these doesn't matter, of course.)
This one could be a little harder, because you need a large graduated vessel. Fill the empty vessel with roughly enough water to submerge the object. Put the object in and record the change in the water volume.