You can't test it, you've got to trust it.
How do you test a guy's love? How can you be sure he'll never leave you?
With a Thermometer !
Right now your just browsing through time.
Choose the things that will mean the most,
and last the longest.
"Testing" something so complex as romantic love is like trying to contain air in a bottomless bucket. What you 'test' today .. Will be challenged tomorrow. Love is liquid that cannot be contained nor measured in difinitive terms .. Because it changes and metamorphasizes as each individual grows as an individual. Love is not something that is measurable .. Because you are always working on it, nurturing it and allowing it to grow. It's a work in progress.
A true test of something as obscure as love is with something equally obscure .. time.
There are few guarantees in life and someone sticking around forever is not one of them. However, if you start playing games, and test his love, I can guarantee he will leave. Not having trust in your partner is one sure way to kill a relationship.
Gain a lot of weight.
I believe from personal experience that you can only test a lasting love by seeing how long it can last by hard work and determination to stay together. There is no other way that I can see. If you make up your mind that after all other relationships you have had, you have learned enough to make this work, and you feel strongly that this one was the one you were looking for all your life, then I'm sure it will last.
You can't be sure he'll never leave you, some marriages fall apart after 40 years. You're insecure and asking for assurances no man can give you.
If you feel you have to test his love then I'd advise against that because what you're trying to do is going to drive him away.