Testing upgrade to PHP 7
At this point assuming your site doesnt make use of extra modules, you can likely assume 7.x php works fine. I have found in numerous sites in the last few months that moving to php7 went fine and most php 7 were squashed in popular D7 modules in the last year or two.

If you have alot of custom modules you could try running a php 7 linter from github to check modules for php7 related issues, rather than trying to live-test drupal itself.

Something such as this github repo:
https://github.com/wimg/PHPCompatibility