We study polyisoprene elastomers reinforced with carbon blacks (CBs) of various grades. In addition to mechanical characterization at medium/large elongation ratios, we use deuterium NMR experiments on stretched samples to measure the local strain within the elastomer matrix and X-ray scattering to measure the onset of strain-induced crystallization in the reinforced systems. We show that NMR experiments and measurements of the onset of crystallization are indeed sensitive to the different degrees of reinforcement observed according to the various CB grades and volume fractions. The measurements show a good correlation between macroscopic (mechanical) and microscopic (NMR, crystallization) measurements. This indicates that the techniques used here are valuable techniques for characterizing reinforced systems in which fillers have complex morphologies and dispersion states. The discrepancies between the results of both macroscopic and microscopic measurements are analyzed. The local strain as measured by NMR is lower than the macroscopic strain. This indicates the presence of some degree of local strain (or stress) inhomogeneity within the elastomer matrix in the reinforced systems. This inhomogeneity is more pronounced in the presence of more reinforcing carbon black grades.