Power to weight is the answer. I have seen many computerized methods to measure the thrust but they all are not perfect, the best way as Scar mentioned is using a digital fish scale. Invest around 20.00 dlls. And you will have a very useful tool. Truth is that if you already have the plane and the engine you will have to work it out in some way but the idea is to have a next better powered model.
When building a kit or just about to buy an ARF model look at the weight, think also that there are in the market a lot of engines of the same displacement but with very different characteristics, just by example two engines that are of the same displacement like the YS 120 four stroke and the old Saito 120 four stroke, they almost weight the same but you will obtain a better thrust from the YS and it can handle a bigger prop. which is usually better for many applications.
As per the RPM you can find that different displacements of engines as gassers can give lower RPM readings than glow powered engines, but you will find a better thrust in those big and heavy gassers.