You discover your own strength.
When James talks about the trials of life he describes them as a test, but not so God can see how strong you are. God already knows that. Pain is an opportunity not for God to see how strong you are, but for you to see how strong you are. If you wanted to see how much weight you could bench press at the gym, the only way to find that out would be to continue to put on more and more weight until you discovered your threshold, your limit. In the same way, pain is the only way to truly discover how strong you really are. And in Christ, you are much stronger than you think.