Running a test sync
If you want to see the sync in action on a limited dataset, you can always apply some intelligent rules to narrow down the sync. Here are a couple of ways to do so.
Adapt rules to only sync contacts with a specific dedicated test-tag, and have it update a specific field on the other side as extra proof. If you only tag one contact in your CRM with this very specific tag, only this contact will sync. The other contacts remain untouched.
Don't have tags in your app? Consider creating a test contact with a dummy or test e-mail address and only leave this rule enabled. Works just as well. Just know that sometimes a dummy e-mail address won't get created inside your ESP client.
Disable rules to prevent contacts from being created into an environment where you want to avoid unexpected contacts from being pushed towards!
Only if contact exists in the other app
Perform a bidirectional sync without actually creating new contacts, but rather, only updating whatever contacts the two apps have in common. Choose the filter "exists in other app" available from the filter menu inside rules.