Telling my tenants that the apartment smells terrible - in the nicest possible way?

[Jon Huber]

So, this goes back to my first experience of being a landlord. A woman (who ironically was a realtor), was renting my apartment. I allowed her to have a pet, as it was a tiny chihuahua. I was suspecting that she was not walking the dog, as the apartment began to smell like the dog was going in the apartment. Long story, short... she was the world's worst tenant and was kicked out and lost her security for something unrelated to the dog. Upon cleaning her place, it was discovered that the dog was going underneath the bed.

How could I have nicely (and more importantly legally) handled the situation?