You can use TEM analysis, AFM and SEM analysis and also CARD analysis to some extent to identify size of nanoparticles. Out of this TEM analysis provide some what accurate results
If your nanoparticles are airborne or can be brought into the air, then a Scanning Mobility Particle Sizer (SMPS) can offer accurate high-resolution measurements from about 1 to 1,000 nm; see for instance http://www.cas.manchester.ac.uk/restools/instruments/aerosol/scanning/.