I don't know if we can tell it for any particular star, but we can reason about large numbers of stars. We have a decent model of how stars must age, given thei

I don't know if we can tell it for any particular star, but we can reason about large numbers of stars. We have a decent model of how stars must age, given their mass and composition; we have a decent model of how that composition must have evolved as the universe ages, so if there were many Dyson spheres around then I think that should be observable as there being "far too much" red dwarfs compared to how many we should observe.