How to check if specific resource already exists in CloudFormation script

I am using cloudformation to create a stack which inlcudes an autoscaled ec2 instance and an S3 bucket. For the S3 bucket I have DeletionPolicy set to Retain, which works fine, until I want to rerun my cloudformation script again. Since on previous runs, the script created the S3 bucket, it fails on subsequent runs saying my S3 bucket already exists. None of the other resources of course get created as well. My question is how do I check if my S3 bucket exists first inside the cloudformation script, and if it does, then skip creating that resources. I've looked in conditions in the AWS, but it seems all parameter based, I have yet to find a function which checks from existing resources.