Depending on how your view finder display is setup, you can see down at the bottom right (when you depress the shutter release button) an (rXX) where XX represents the number of shots before the buffer runs out. If you're shooting continuously and you see that rXX number get to zero then it's likely a buffer issue. You won't be able to shoot more until the files are written to the card. Typically when you hit that wall your actuations slow to maybe 1 shot per 1 second or slower. Otherwise I would also check your focus options like kevy73 mentioned.