My cousin's two year old boy fell over the back of the loveseat this morning. He went crashing face first to the floor. He bled on/off all day. How do we get to know if he has a broken nose?

You don't. Most 2 year olds are very flexible and a broken nose is unlikely but only an xray would tell for sure. A doctor may get an idea from the exam and amount of tenderness on palpation and movement. Make sure he can breath through his nose and there is not undue swelling and look for any devaition. If in doubt, check it out. Sorry for slow response.
Nose crooked? Try icing the nose for 10' every hour for a day or so to reduce swelling. Use afrin otc (oxymetazoline) for nose bleeding. If the nose looks crooked you have a week or so to get it evaluated by your local ENT or facial plastic surgeon for correction (if needed). Good luck!