I think you should differentiate between the mechanism of evolution and the historical evolution. On the one hand not only the basic mechanisms of evolution (variation by mutation, natural selection...), but also the whole mechanism of evolution can be observed. Not nearly as easily as gravity, but it can be observed. (Maybe here the example of antibiotic resistance would be a better, more verifiable example than the fruit flies) Still the mechanism of evolution is only a theory explaining observations which were intuitively called 'evolution'. Those observations are facts (here you got evolution as a fact). The explanation by the mechanisms of evolution is not. It is a theory by definition. On the other hand the theory of historical evolution explaining the variety of species can't be observed, because it is supposed to have happened in the past. You can only observe Fossils and the current flora and fauna. Then you can try to explain how all this came into being; for example by assuming a random abiogenesis of a single cell and then applying the mechanism of evolution. This random abiogenesis and the following evolution over millions of years is what a layman thinks of as 'evolution'. This certainly is 'only' a theory. It isn't and never will be a fact. It is very importent to clearly differentiate these things. Strictly speaking even the assumption that everything always falls downwards cannot be considered a fact. How can you know that there will never be a stone being observed falling upwards?? This may seem a bit like I'm splitting hairs, but if you don't strictly apply well defined terms of 'fact' and 'theory' you will end where we where a few centuries ago considering it a fact that we live on a flat earth. Don't ever hesitate to present things very differentiated! Maybe referencing the critical rationalism by Karl Popper would be a good idea, too.