I believe in life on other planets, but I don't think we've been visited by representatives of other intelligent civilizations. The greatest barriers are space and time and these barriers are generally enormous. Aliens would probably have to cross oceans of time and space to travel from their planet of origin to ours and I just generally believe that it's too big an obstacle. Even if it's not impossible, it is likely to be impractical and so if there are civilizations that colonize space, it doesn't necessarily mean they would travel very far in cosmological terms. If it takes you 50 000 years to get to one planet and 500 years to get to another, the choice seems simple. Still, colonizing space would make a civilization more resistant to dying out and they could radiate out over time, but still ..
There's also a lack of evidence that we've ever been visited. I imagine the only real point of visiting our planet would be because it holds some resource they could use, even if it was just something simple like decent gravity, water and a good temperature. When we look to other planets, we think of whether it's a place we could colonize or perhaps harvest something from .. I assume intelligent aliens would have the same practical sensibilities.
If intelligent aliens had visited earth in the past, for example when the dinosaurs were around, I would think they'd build a colony - perhaps bring some alien organisms for agriculture or whatever. There'd be some indication they were here, like a very different lineage of organisms that suddenly seemed to appear in evolutionary history or perhaps even ruins of an ancient civilization.
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