Yes. I've actually performed the former experiment.
Plus how do you measure total energy output? Is some output in form of heat?
A nuclear reaction experiment might measure heat output. If I'm doing a particle experiment such as I described, I'm going to measure energy with an electromagnetic shower counter, also called an electromagnetic calorimeter.
There are many kinds of shower counters, but they all use the same basic principle: high energy photons interact with matter to create electron-positron pairs, which emit photons that create more e+e- pairs, and so on, and so on, and so on, until the photons are too soft to create new pairs. So I might, for example, have a series of lead plates, interspersed with scintillating plastic. This special plastic emits light when charged particles pass through it. I can then measure the light output with photomultiplier tubes. I can measure the energy of the photon by counting the number of particles in the electromagnetic shower.
Plus, when you have isotopic decay, do you have a loss of mass?
Oh, yes. This you measure by looking at the masses of the nuclei, and simply subtracting. The lost mass--maximal for iron--is known as the nuclear binding energy. It is very well known.