If you find what you believe to be a lost pet, bring it to a reputable animal welfare organisation or veterinarian, where one of the first things they will do is us a radio-frequency ID scanner to detect the presence of an animal identification microchip.
If they find the microchip and read its identity number they will then be able to search the microchip databases to find one that holds contact details.
If those contact details are up-to-date then the pet can be returned to its keeper.
Before registering the animal as lost please just check whether the pet has an identification tag with contact details on it. Sometimes this might be written on the inside of the collar tag. If it does, try contacting the keeper. If the lost pet does not have an identification tag, please take the lost pet to a local vet or welfare centre who will have a scanner and be able to find the microchip number and search for databases for the owner's details. You could also contact your local authority who have responsibility for dealing with stray dogs.
You should call as many vets near where you found the animal to leave details about the pet in case the keeper has also notified them that it is missing. It is also helpful to contact rehoming centres and welfare organisations near you to leave the details of the animal with them.
Here are some useful links: