If you are looking for a reservation recently entered or coming up soon, the best way is in
Reservations /
upcoming reservations or
latest reservations entered.
If you're looking for a reservation for a specific date/time, the quickest way is in the reports&schedule tab (top items: day/week/month). On the right-hand side you can switch between day/week/month views and on the left-hand side you can navigate to a different time period.
If you're looking for a reservation of a specific customer (by their first or last name, login, user ID, email address etc.) or if you know the reservation ID, you can quickly do this in
Reservations / Search.