Some can be easily spotted by visual inspection for being bulged just like the one in the linked article photo, others will need to be measured. Leakage (ie, w

Some can be easily spotted by visual inspection for being bulged just like the one in the linked article photo, others will need to be measured. Leakage (ie, when a capacitor exhibits resistive properties by conducting current also when fully charged) can be measured easily by using an analog tester or a digital one plus a few parts, while the ESR (equivalent series resistance) measurement would need a proper instrument called ESR meter. The ESR is an unwanted static series resistance the capacitor would exhibit no matter the frequency it works at, it increases with aging and it's independent from the capacitive reactance which it's related to the frequency. Therefore, the lower the ESR, the better quality or condition is the capacitor. However, you can just ignore the 2nd sentence if repairing a device would require just a bunch of capacitors; quality ones aren't that pricey, and if you find a bulged one, it is often advised to replace all capacitors employed in the same stage anyway. Some in fact can degrade without showing any visible hints of their condition.