On the Etihad website, once you have selected your flight(s), click the link "View Fare Rules" below the total price:



This brings a pop-up screen like this one:



The first letter of the Fare Basis Code is the fare class - in this case, "Q".

The various other online travel agents vary - some clearly display the fare classes, others allow you to discover it though it may not be obvious how to do so, and others do not disclose the fare class at all until after you have purchased and paid for the ticket.

Beware, however, that you are never guaranteed that you can upgrade your flight, so consider this before you pay extra for a particular fare class (if, for instance, you are considering buying a more expensive ticket than those offered to you in th cheapest available L,U,V,E fare classes. I could be wrong, but I expected that the L,U,V,E fares are the "Economy Saver" fares, while choosing "Economy Value" or "Economy Freedom" will always give you a fare class from those with the lower mileage upgrade requirements).

There has to be a free seat available for redemption in the business cabin before they can allow you to use your miles to upgrade. On very busy flights, this may not be the case. As a last resort, check with the airport ticket desk (or at checkin) in case any unsold seats may be available for upgrading with miles just before you travel.