When viewing the activity in the system, you can tell if your activity synced or not by looking for the cloud icon, like the below screenshot. If it is filled in and colored (purple/blue), then the activity did sync. You can view this on the tasks, outbox, meeting agenda, and calls page. You can also click on that same cloud icon, and it will take you to the task that we synced and created in your CRM.
If you do not see that cloud icon filled/colored in, and is just gray, then it did not sync.