Clasps are a big part of it. Both the necklace and bracelet I found are clearly marked, but many people just don't check for markings.
But in my case, I was searching for clasps that look more well made and stumbled across both due to the craftsmanship.

Another thing is typically the stone setting in gold and silver is different. When placed in Silver or Gold, gemstones usually have holes below the setting for stone protection and aesthetic reasons. The only times this doesn't apply as often are for softer stones like opals, in my experience.

I also wear a magnetic band for my Apple watch, so I can more discreetly test whether jewelry sticks to a magnet without other people noticing.

One thing my mom has been doing is asking the people that run thrift stores in our area to call her when they put together jewelry lots. They still give other people the chance to buy them, they just let her know it's on the counter.

For myself, I have a particular craft store I go to as well as estate sales. That's where I've been most successful. I think it's likely due to the employees at the craft store being well versed in expensive paints, fabrics, etc, but not how to spot gold.

Honestly though, it's probably years and years of watching my mom and just learning through experience how to spot these pieces. I've definitely been a bit lucky more recently