When a woman breaks the contact barrier during a conversation, it is almost a sure sign that she's interested. It can be as obvious as touching your arm or knee while making a point, or as faint as having her knees come into contact with yours under the table. But you must make sure that she makes the first skin convergence.
A less direct way is if she mirrors your body language, which is something women do subconsciously. When you lean in, she leans in. When you rest your elbows on the table, she does the same. Duplicating your actions is her way of showing you that she's "in-synch" with you.