If an oxidation layer forms between specific device's contacts and the terminals of its battery, are there visible signs that it happened?

yes... corrosion of nickel plated steel (battery terminals in disposable batteries is usually blue-ish white powdery looking stuff

https://s3-production.bobvila.com/articles/wp-cont...

https://i.pinimg.com/564x/d0/14/e3/d014e37a2c0a259...

corrosion of lead battery terminals is similar by usually more blue in color

http://autotrends.org/images/battery-terminal-corr...

corrosion of the leads can be anything from blue-ish white to rust colored

https://encrypted-tbn0.gstatic.com/images?q=tbn%3A...

https://i.imgur.com/tA5yc.jpg

corrosion of electrical contacts in a circuit can be any of those colors or a blackened pitted surface depending on the materials of construction

https://upload.wikimedia.org/wikipedia/commons/thu...

oxidation is rust.