looking for device that checks whether door is locked

[curious]

Does anyone know of a device that checks whether the door is locked ?
Must be cheap and easy to install.

Note : I don't want to check whether the door is closed. And i do not want to completely replace my door lock (cilinder)
Hardware from Nuki etc are far to expensive.

[Egregius]

I use a Fibaro door contact with a microswitch installed inthe hole where the lock goes in.

[K3rryBlue]

lots of options:

• microswitch
• reed contact (need to fix a small magnet to the deadbolt)
• hall element
• deadbolt sensor
• Insteon 2845-222 Hidden Door Sensor