The Spanish region of Catalonia recently announced it would fine drivers using the popular app up to €6,000 ($8,000), which taxi drivers argue threatens their livelihood.
Now police in the region's capital have been given instructions on how to act when they think a driver may be illegally picking up passengers, Spain's El Mundo newspaper reported on Wednesday.
Police will have to separately interview both the driver and the client in the vehicle, those instructions state.
Agents will also have to investigate what type of arrangement the driver of a vehicle has with the firm offering the transport service, what sort of contract the driver has with the passenger, and how any payment is made.
Police are also required to take photographs or screen captures of mobile phones as evidence a taxi app like Uber has been used.
Drivers who are found to have illegally used their private car to pick up passengers will also have their car impounded, the instructions state.
Taxi drivers in Spain have protested against Uber in recent weeks, as have drivers in France, Germany and London.
But the Vice President of the European Commission Nellie Kroes has responded to the strikes by saying that technology is not the problem, and that a solution needs to be found.