As per the question - how to I test that a knife is sharp enough? And/or, how do you know when a knife needs sharpening?  Edit: Yesterday I attempted (I feel quite successfully) to hone my knives for the first time ever (requiring me to purchase a honing steel), which immediately made a rather noticable difference (I shall be honing them regularly from now on). Not sure whether additionally sharpening them as well would be a good idea? The suggested tomato and paper tests make sense, and I shall try them in the near future and see.. 
I usually notice when cutting onions and tomatoes. With a very sharp knife cutting an onion doesn't cause much tears at all. As it dulls though it does more crushing than slicing which releases more gas into the air, which makes you cry more. Ripe tomatoes help because they are so tender. If it becomes difficult to slice a tomato without crushing it, your knife is too dull.  A well used, properly cared for knife should require sharpening every 6-18 months, depending on usage. Mine get sharpened yearly.  You may find it beneficial to read these related questions, and their answers:   How can I safely improve my cutting technique  How should I care for my knives? Why not put knives in the dishwasher?  