Force it below the surface of water and measure the force required.
Mass divided by Volume = Density. Or use the displacement method for an irregularly shaped object.
Weight it. Find it's volume. (hint - how much water does it displace). Divide.
Finding the center of gravity of an irregularly shaped object can be done using the plumb line method.
If the object in question is water proof, and small enough. You can measure the amount of displaced water when it is placed in a tank full of water, this may not work for a lot of things but it can help when attempting to measure some objects.
the water displacement method
To calculate the volume of an irregularly shaped object, a good idea would be to get a bucket full of water and submerge that object into the bucket. Then measure the amount of water that runs over and that should be the volume of your object. For example if you take a sealed bottle of bottle stick it in a bucket filled with water, then let it's volume filll the space and displace the water. The water that is displaced or the water that runs out is the volume of your irregular shape. Get it?
Asteroid
The amount the water rises is dependent of the volume of water displaced by the object - thus it can be used to measure the volume of the immersed object. If the object did not immerse completely - if it floated - the displaced fluid could instead be used to calculate the relative density of the object - when combined with the total volume.
Calculate the volume of the substance by measuring the length, width and height. The formula looks like this: Volume = L (length) x W (width) x H (height). Determine if the object or substance is irregularly shaped. If so, you can determine the volume by finding out how much water is displaced when the object is dropped into a beaker of water. Calculate the density once you know the volume and mass of the substance or object. Density is equal to the mass of the substance divided by its volume: D (density) = M (mass) / V (volume).
Tight
Use volume displacement method.
Because the L, for example, will not be the same at all points along the irregular object's W or H. And similarly with the other two measures.