The answer is: you don’t!

You hire an expert to spot a forged signature. You hire an expert to go to court, talk to the jury, and confirm whether this is or is not a forged signature—this is a striation, this is a dot, and I’ve been doing this for 20 years.

Some people think they can spot a forged signature and that it’s easy. Sometimes, they literally google how to forge a signature. No one’s going to teach you how to forge a signature because that is a crime, so you’re going to have to attempt to do it on your own.