It depends on what information you have. If you know the volume of
the object, and the object is completely submerged: then volume
displaced is just the volume of the object. Sometimes you can figure
out volume displaced by employing Archimedes' principle and/or static
equilibrium. For example, suppose you have something floating on
water, partially submerged. You know its weight, although
maybe you don't know the volume submerged (which is the volume
displaced). From Archimedes you know that the buoyant
. From static equilibrium
you know that the weight is equal to this, i.e.
. We can then get that
For other problems, depending on the details,
you may be able to set up various equations to get displaced volume.