In general, web sites will tell you in the Content-Type header what kind of data they're returning. You can determine that by getting the ContentType
property from the response.
But sites have been known to lie. Or not say anything. I've seen both. If there is no Content-Type header or you don't want to trust it, then the only way you can tell what kind of data is there, is by reading it.
But then, if you don't trust the site, why are you reading data from it?