You can try squeezing the nipples a few times to see if there is any milk coming out. Also, your breast would feel soft and somewhat "deflated" (for lack of a better description) when it is emptied. As a guide, it is recommended to let your baby nurse on one side for at least 20 mins before switching. Though I think it may be different for every baby, so this is just a general guide with regard to the amount of time a baby should be nursing on one side.
For latching, you will feel your breast turns softer compared to before latching. If your baby doesn't drink much, you can try to pump out after latching to empty your boob so to stimulate more milk supply.