You literally can't tell. You can tell by touching or by extreme close inspection if you have had held examples of both by subtle process/cultural differences, or by racking if you have done so in the past in either or both, due to major internal differences. But if you think toy guns are like nerf guns painted in black, then you just can't. It's not an easy tell like wrong corner rounding on Apple lookalike aluminium chassis.

Maybe I would change my mind if I were an actual Uber driver, but I'm thinking that I don't care if my passenger has a gun, as long as they're being safe and not pointing it at me. I've been around guns. They don't scare me. My job is to drive them from point A to point B. Why should I care if they are packing, or if they're underage drinking a beer, or if they're texting a mob boss about money laundering?

A human driver can't tell. I've had gun guys mistake my airsoft guns for real ones. Average person who isn't familiar with both real guns and airsoft has no chance.