You can actually preview them with Calibre's built in thingie. After converting, it will say on the main page something like "epub, mobi" etc (all the formats you've put it in), just click on the mobi and it opens up in the viewer.
Anyway though, it seems to be depend on the epub. I've had problems with bought epubs (from Baen and Fictionwise), but what I did was load them up in Sigil, save, then use Calibre and that fixed the problems.
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