Do the police check your mobile device during police checks?

They cannot legally check it without a warrant.

Depending on the circumstances of the arrest/check, and the method of seizure of the mobile device, you are subject to a certain set of rights, laws, or protections. First off, know that it is your right to decline the warrantless search of your mobile phone. If you are arrested or taken into police custody, you should verbally state that you do not consent to a search of your devices. A law enforcement agency is only permitted to conduct a warrantless search of your device if a compelling case for an emergency can be made.

If the authorities are using evidence obtained through an unlawful search of your mobile device against you in a criminal proceeding, you can move for that data to be suppressed under the Fourth Amendment right to freedom from incidental seizures.