Saarinen table top...  

ebay and craigslist
The places to look for tulip tops, ebay and craigslist. Post a photo of the base and it's easy to tell a Knoll from the vintage knock offs.

I don't have a photo
of the table base. It's belongs to my best friend's father. He lives in Baltimore. I'm going mostly on hear-say here. Neither one of them are very 'with' it. They don't own digital cameras and aren't very internet savvy anyway. I'll get her to describe the base in more detail. What questions exactly should I ask her to determine if it's Knoll table or a knock-off? His parents did own some original Knoll stuff including a womb chair, so it's highly possible that this is Knoll too.

Knoll or knot
The easiest thing to look for: If there are any holes in the base for screwing on a top then it's not Knoll.

Rules of Engagement
Do these same rules apply to the tulip tables?

knot not
If there are screw holes in the metal base for attaching the top it is not Knoll. This simple test eliminates most of the vintage 60's 70's knock offs.