If you have them in a glass capillary mixed in water (seal both sides of the capillary after filling it), they will start to either go up or down the capillary due to Stokes' Law (http://en.wikipedia.org/wiki/Stokes'_law). If you set a camera or microscope from the side view, you'll be able to find the velocity of these particles as they go either down or up. Using the velocity and the radius, based on Stokes' Law you can find the density of these particles.

You can measure the osmotic pressure of the suspension, to get the compressability. Simplest way is to dialyse the suspension against a high molecular weight polymer solution such as polyvinyl pyrolidone, and then either calculate the osmotic pressure from the polymer concentration and virial coefficients or measure the osmotic pressure using a vapour pressure osmometer. Then measure the particle concentration simply by drying the gel particles.

see paper SIMPLE DETERMINATION OF THE OSMOTIC-PRESSURE OF STERICALLY STABILIZED COLLOIDS

By: COSTELLO, BA; KIM, IT; LUCKHAM, PF

JOURNAL OF THE CHEMICAL SOCIETY-FARADAY TRANSACTIONS Volume: 86 Issue: 21 Pages: 3693-3694 Published: NOV 7 1990

You could also get an idea of this by measuring the viscoelastic properties of the suspension by measuring the elastic modulus as a function of particle concentration. 

See paper CONCENTRATION-DEPENDENCE OF THE LINEAR ELASTIC BEHAVIOR OF MODEL MICROGEL DISPERSIONS

By: EVANS, ID; LIPS, A

JOURNAL OF THE CHEMICAL SOCIETY-FARADAY TRANSACTIONS Volume: 86 Issue: 20 Pages: 3413-3417 Published: OCT 21 1990

 Thank you very much Mr. Salil and Prof. Luckham for valuable comments!

You may also try to determine water content and then calculate the hydrogel density