You can try this: before reaction, take a spectrum for the reactants. then let the reaction starts, if the reaction is slow, you can take out some sample and do an NMR measurement, you may see some new signal coming out, the integration of these new signals represent their populations. As the reaction gets more complete, the intensities of the new peaks get higher and higher. But this method applies only for slow reactions, it should be slow enough to let you take some sample out (more than 5 mins I say).