Use AWS IAM Policy Simulator with the CLI

aws-iam-simulate.sh

# The action we want to test
_action="s3:GetObject"
# The JSON you want to convert to a string
_json_to_convert='{ "Version": "2012-10-17","Statement": [{"Effect": "Allow","Action": "*","Resource": "*"}]}'
# Your resource arn goes here
_resource_arn="arn:aws:s3:::${_some_s3_bucket}/${_some_prefix}/*"

# This command will format the json policy as a string for the cli to use
# echo ${_json_to_convert} | jq '[.|tostring]'
# But we will send it to a fifo with '<()' instead of creating a json file every time

aws iam simulate-custom-policy \
--policy-input-list \
file://<(echo ${_json_to_convert} | jq '[.|tostring]') \
--action-names "${_action}" \
--resource-arns "${_resource_arn}"