Bardic Knowledge overlaps a number of knowledge skills. Most notably, Knowledge(history) and Knowledge(local). The check is 1d20 +bard level +Int. Actual knowledge skills improve much more quickly for the average character. Because of this, I tend to be very liberal with what Bardic Knowledge applies to. I let bards substitute it for any knowledge skill when trying to find out something that would be worthy of song, poetry, or prose. The "worthy" part is the tricky bit. It requires a case-by-case ruling. It's unlikely, for example, that a song would give enough detail about rutebegas to identify one on sight. However, a song about the poisoning of a king might include poetic descriptions of the leaves of the deadly nightshade plant that was used to make the poison. In such a case, the bard might be allowed a bardic knowledge check in place of a knowledge(nature) check to ID the plant. This example is extreme, and a little wierd, but you get the idea. If you think the information is likely to be immortalized in song, then a Bardic Knowledge check might reveal it.
As far as making more than one check, I would say no. But I would allow a +2 synergy bonus on the Bardic Knowledge check if the bard had 5 or more ranks in an appropriate skill. But not the other way around, because that could give the bard an effective bonus in just about every knowledge skill.
I hope that helps.

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