If you are syncing all of your leads to Salesforce and want to look at those with sync errors, I would create a Smart List using the filter "Not Lead was Synced to Salesforce" with Assigned to is "Empty".
This should show you the list of leads that have not synced to SFDC. From here you could view the activity log of each lead and view the Sync Lead to SFDC Activity Type and see the fail sync reason. This list is going to show everyone who has not synced to Salesforce, so if you do not push all of your leads to Salesforce those will also appear on this list.
The failed sync reason in the activity log should make it easy for you to see if there are common failed sync reasons that you can further troubleshoot.
Hope this helps.
I agree. I do it that way. However, if you want to zero on specific error, going to each record one by one is very consuming. Is there any other option?
There is no way to run an en-mass report of the errors. The best you can do is have a report of the leads that have experienced the errors, and looks at them one by one, identifying the errors and troubleshooting. If you're good, you'll eventually get to them all.