There are several ways to see trips that have not yet been approved.  One way is through is through the Calendar.  The calendar is helpful if you are most concerned about trips within the next few days because they can easily be spotted on the calendar.

Click on the Calendar tab:

Click on the Trip Calendar from the left menu:

Trips that are Pending will show in orange on the calendar and will also have a "Pending" indicator:

Another way to find a list of trips that have not yet been approved (Pending Trips) is through the Trip Inquiry.

Click on the Trip Approval Tab

Click on the All Levels - this will show trips pending for any level of approval:

Choose "All" for the Next Approver name and leave the other filters set to "All" as well:

The resulting list will show trips still pending some level of approval and which level it is waiting on and who the next approver is: