To check whether a guest's device is actually rooted or not, have your guest download the following apps from Playstore and run a safety test. Once the test is done, please ask them to share the screenshot of the message displayed as that would confirm the result of this test.
Please find the link below:
https://play.google.com/store/apps/details?id=org.freeandroidtools.safetynettest
https://play.google.com/store/apps/details?id=com.jrummyapps.safetynetchecker