This depends on which structures created by the humans are left standing. We are talking about cities here. Skyscrappers. Mills, factories, production units. Libraries.
How many (and in which condition) of these are standing? And where, too? Long after the human race (I'm counting 50,000 years here) is extinct, an alien spaceship lands on earth. Even after 50,000 years of storms, cyclones, rainfalls, scorching heat, meteorite impact events and earthquakes, you would be amazed to know how much of our current structures would be immediately detectable. Not detectable as in "oh wah! look this is the statue of liberty!" but as in "here are some fragments of a large figurine. most parts are missing but here is the right foot and some portion of the left shoulder."
With dating methods, they can easily date the remains and know that an advanced intelligent race dwelled on this planet nearly 50,000 years ago. They could excavate a museum and find some fragmentary pieces of earlier civilizations which would date to 55,000 years or so (egyptians). In regions where fossilization conditions are favorable, they might excavate human skulls and limbs and form a good understanding of our anatomy.
This is if the alien civilization was smart enough and was technically advanced enough (nearly our level with 10% margin).
If they don't have any dating methods available, then they can definitely deduce that a very advanced (they don't know dating techs so definitely our current tech progress is greater than theirs when they land) race used to live on this planet. But they will not be able to deduce when they went extinct.