Wiki

A failing electrolytic capacitor can often be recognised as a build up of internal pressure may cause the top to bulge, or the capacitor no longer to sit flush with the board, or electrolyte to leak from the bottom. At this stage it probably won't be performing well, causing the equipment to malfunction. If not replaced, it may even explode. However the fact that an electrolytic capacitor shows no visible sign of deterioration is by no means a reliable indicator that it's good.