How to Measure the Mass of a Neutron

The electron neutrino doesn't have a definite mass. It could just be like something light moving fast or something heavy moving slow. And what we're looking for is the maximum scenario. Like, are there any cases where the electron takes all of the energy available? There's like a certain energy budget for producing this, subtract out the electron mass. Then we wonder, like, does the neutrinos take a cut? If the neutrino takes a cut that leaves a smaller budget for the electron and you'll never see an electron having energy higher than that limit.