I mean the cumbersome gold standard is literally just using the program. Fictional dialogue:

  A: Is it possible to pay for a product on Firefox mobile?   
  
  B: Yeah, I manually test it for each new version  
  

Please let us not forget: any automated solution is just there to reduce the amount of manual testing needed.

Of course that is a lot of work and testing every possible combination of situations, technologies etc is probably not feasible, but you shoud have a checklist that you follow, if there are tests you can't automate.