Which is exactly what you want to do. It's called the "scientific method". To determine if your case cooling is inadequate you need to provide a condition where you get more air circulation. By adding more air flow over the cards, if nothing changes, then you have now determined that increased air circulation will not change anything and the card must be the problem. We also use a fog machine to diagnose air circulation issues, but would never bother with that until the fan test is performed.
http://www.vornado.com/circulators/533-Compact-Circulator
http://www.amazon.com/Chauvet-Hurricane-700-Fog-Machine/dp/B0002D0JX8/ref=sr_1_3/186-3150029-2630059?ie=UTF8&qid=1454689014&sr=8-3&keywords=fog+machine
If you do see a significant drop in temps, then it is a clear indication that your case cooling is inadequate. Adding more and / or switching to case fans would be the next logical step. If the problem persists, then localized stagnation is the likely cause (here's where the fog machine comes into play) and this can oft be resolved with a MSP fan mounted on the back of the HD cage [providing localized air circulation to the area around the GFX card(s).
It's always better to determine what is the cause of the problem before investing time and money in the wrong solution.