There is a simple dye test to see if a toilet is running. This test shows whether the flapper is seated correctly. Use red, green or blue food coloring or flavored drink mix.
If you use cleaning tablets in your commode tank, remove them and flush the water clear. Take the lid off the back of the toilet. Put the coloring in the back of the tank, but don't flush the toilet. Let it sit several hours.
If color shows up in the toilet bowl, the flapper seals in the toilet are not seating properly. A worn flapper is the most common cause. Get an inexpensive replacement to fix the problem.
If you have to jiggle the handle to make your toilet stop running, a handle or flapper replacement might fix it. If you suspect you have a leak, contact a plumber or your landlord.