It would seem to me that I don't have to have ever witnessed a particular phenomena to be able to recognize evidence of it. For example, if I were to see a set of footprints in the sand, and on every left footstep there's a small hole, I might explain this finding by hypothesizing a person walking across the beach with a nail stuck in their shoe. Of course I understand that it could be explained in an other way, but if that was actually what had happened and I'd never before seen a person walking with a nail in their shoe then I'd have recognized evidence of something that I'd never seen before. But I have experienced footprints and nails before so perhaps I'm mistaken.
My question then is, Is it possible to recognize evidence of something I've absolutely no experience whatsoever of? And what are the implications to the idea of sense data being evidence of the external world (if our only evidence of the external world is our sense data, how can we hypothesize an external world to explain such data when we've never experienced it...if you catch my drift)? Wouldn't we then be using the very thing that we're attempting to explain as the only evidence for the explanation?