Cosmology.
Modern astronomy observes the events of things which occurred in the past. Looking into the heavens is like looking back in time.
By applying his equations, and the resultant predictions thereof, to the study of cosmology.
Any theory is only as good as its predictive power.
It took 100 years for physicists to confirm all of the predictions emanating from Einstein's equations...many have tried to disprove the predictions, but each and every one has held true due to observable phenomenon.
The speed of light has always remained constant....and could NOT have ever changed due to E = mc^2....this is one thing that utterly destroys a YEC worldview...as they like to fantasize that the speed was different in the past in order to accommodate their incorrect interpretation of scripture.
Your smart phone. Did you know that Einstein's Relativity is what makes it work?
We didn't put a man on the moon by believing in a young Universe, etc, etc, etc...