I've had people incredulous at time dilation effects in relativity. I give them the example of this having been directly measured in the 70's. There, they put one clock in the lab and another on a jet and flew it around a bit.

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

Also, high-resolution GPS requires SR and GR corrections to work. This is a working technology.