travertine tops keep looking as great as they do

We bought a travertine coffee table and side table a few months back and are concerned about making sure the travertine tops keep looking as great as they do now. We always use coasters, but recently had a guest who (despite a dozen or so coasters being spread out around the table) put a wet beer bottle directly on the coffee table. Although it had only been sitting there for maybe 15 to 30 minutes before I discovered it and cleaned it up, it caused a ring. Fortunately I was able to remove to the point where it's hardly visible any more. After that happened, we were thinking that maybe we should try to seal the tables, so I bought a bottle of StoneSpecific sealer for tumbled/honed travertine. Before I try to use it, I was hoping to get answers to these questions:

• I think our travertine top is the honed variety, but how do I know for sure? The top is smooth, but not to the point where it is glossy.
• I'm not sure whether the table might have been sealed at the factory. How do I know whether it's been sealed or not, and what would happen to the stone if I tried to seal it after it has already been sealed?
• To what extent would sealing prevent future stains in this type of material?

Thanks, Judith