Depends on how exact it should be. I did an exercise on bread/baked goods quality control in uni once. You an either use a laser scanner to get a 3D model of whatever youre measuring (this is nice, because you can also get Information about coulour, but the scanner is expensive, you can measure pretty much anything though) or submersion in plastic beads / seeds (measure the volume before and after) this is extremly cheap and reasonably exact.

If it's a foam like whipped cream or stuff like cereal puffs: use a beaker and weigh But be aware that this is not the real density, but bulk density. You can look up some constants about which geometries have which void space in bulk and get an approximation for the density of the bulk good from the bulk density.