The easiest way to check if your toilet is leaking is to remove the lid on the tank and look to see if the flapper at the base of the tank is closed, there is no water flowing into the overflow tube when the tank is full, and that the chain attached to the flapper is not tangled up or caught under the flapper itself.
If the flapper appears to be closed on first inspection, flush the toilet and watch to see if the flapper drops completely while the tank refills with water. If it appears the flapper has dropped completely, add several drops of dark food colouring into the tank. Wait for a least 30 mins, and if you see any trace of colour in the toilet bowl afterwards then there is a leak. Most likely the flapper is not forming a water tight seal and it will need to be replaced.