How to determine if a drain running into concrete slab has a trap?
Borescopes on Amazon

The cheap and easy way - purchase a camera and look.

Most newer USB-OTG capable phones will work with USB Borescopes/Endoscopes/webcams.

I am using the same USB scope that I bought years ago for use with my laptop, as a borescope for my Samsung S7 phone.

Paired with the free app IP Webcam and/or native Android WiFi screen casting, it has proven quite indispensable in contrast to how little it cost to purchase online.