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I always just look at the units of the other components of the problem; if those or the answers are in L or atm or mol or K I do .0821, but if I see anything with J it's probably 8.314 since that constant has to do more with heat/work problems.
R is usually (0.08) for most of the equations, except in thermodynamics it's actually 8.3. If you see an equation corresponding with thermodynamics it's 8.3. After you do a few problems it becomes second nature which one to use. Trust me.