You need to get there early. Many good comic book collections are snapped up quickly. Also, talk to the sellers and ask if they have any hidden gems.
First, arrive at the garage sale as soon as it starts. Then, look carefully through boxes and stacks. Sometimes, you can ask the sellers if they have any specific comic book series you're interested in. Another tip is to check for comic book storage containers or special sections dedicated to them.
To find comic book collections at garage sales, keep your eyes peeled for signs of comic-related items. You can also make friends with regular garage sale-goers who might tip you off. And don't forget to negotiate the price if you find something you really want.