If you have vRealize Network Insight in the environment you can just use the "nsx firewall rule where flow is not set" query to see all rules that haven't had a flow match them in the time period you're looking at.
Yes as lhoffer said, if you are using vRNI you can query for firewall rules that are not used (there’s no traffic going through them) as below:
This will be a workaround showing the NSX firewall rules where flows are seen by removing the ’not’ operator. Keep in mind you need to have NSX send the IPFix traffic flows to Network Insight for this to work as per the following: NSX Distributed Firewall Inactive Rules
What is the difference between Hit count and Flow?
In the graphic I see a high Hit count, but 0 for the flow count.