How do I know if an item is still available?
Here you have multiple options. The best thing would be to contact the poster, which can be done through the 'Request' button located on the product page.
Items that have been Given should not be present, as the Giver should have 'Closed' the item. That being said, there are many occurances where this does not happen.
Also, if you're not getting a response you can check the date of posting on the product page. If it is more than two weeks old, then the product is more likely to have been taken.