There are 2 ways to check which contacts you have saved to a sequence.
Check Contacts Within a Sequence
Launch Apollo and hover your mouse over Engage. Click Sequences.
Click the Sequence you want to look at.
Click Contacts to view the contacts in your Sequence.
Check Contacts With the Search Function
Launch Apollo and click Search.
Click Show More Filters.
In the Filters modal, scroll to "Engagement Activity" and click Sequence to expand the accordion.
Click the Include Sequences drop-down and click the Sequence(s) that you would like to include in your search.
Click Apply Filters.
The contacts in your sequence are now visible.