How to spot rust

[Jonlopes]

Greetings,

I am going to be looking at a 1971 C10 longbed this afternoon. I was previously going to be getting a 69 c20 but the thing was rusted through pretty bad and I decided to keep looking.

This truck looks good in the Craigslist pictures and the owner says it is a rust free California truck. I know that rust in the cab corners and/ or the rocker panels was a standard feature but it sure would be nice if true.

My question is if there is any easy ways to spot bad rust repairs, I do not want to spend $5k+ for a truck and have a bad bondo repair job come back to haunt me. Thanks!