My dog started to pee in the house again

You know by taking the sample to the vet. If the dog has an infection, they will find it.

Is your dog urinating without seeming to know it? If the urine sample comes back negative, a lot of time they will think towards incontinence,which is more common in older, spayed female dogs.

As Sariss said, the vet will be able to test and see if there is an infection.
To save time get a specimen jar from the drug store and take a urine sample the morning you go to the vets.
You will probably have to follow her around and quickly put the jar under her when she goes.

The other option is incontinence. There are a few different forms. Some cause a constant drip of urine as the muscles just aren't strong enough to hold it in and it flows freely.
Others can cause the dog to 'leak' when very relaxed or sleeping.

Final option is behavioural.
Has anything changed around the house lately? More visitors than usual, new pets, being left alone longer etc?