Seatcounter.com can give you an idea of how full a flight is, but do bear in mind that the numbers given are the numbers that the airline is willing to sell in any one block, not the remaining seats on the plane. Looking at it now, the MIA flights on those dates are very busy!

If you do find it's very full, then do mention to the check-in or gate agent that you're aware it's a busy flight, and that if they're looking for volunteers at any point, you're happy to put yourself forward.

However, do be aware that, since the introduction of EU 261/2004 rules on flight delays, EU-based airlines are becoming a lot stingier in their offloading compensation, often going to minimum amounts stipulated by the regulations, or finding loopholes therein to ensure people get nothing.