Electronic – Determining the correct polarity of a supposedly wrong polarized capacitor

As clabacchio said, some capacitors are unpolarized, so it's perfectly fine to put positive and negative voltages on them.

However, it is still possible to put a AC signal thru a polarized capacitor. This is done by adding a DC bias of at least half the AC peak-peak voltage. The entire signal is then still positive, but AC-wise the capacitor acts on it normally.

Nowadays, polarized capacitors are mostly used for bulk storage on power supplies to reduce ripple and to provide short term high current. A nominal 12V supply, for example, may have 1Vpp AC ripple on it. That means the voltage is from 11.5V to 12.5V, which is fine for a electrolytic or other polarized cap. However, the large value of the cap will be applied to providing current to counter the 1Vpp AC voltage swing.

Put another way, polarized caps must always have a positive voltage on them, but there is no such restriction on current.

On those little plastic rectangle surface mount packages, which are most common for Tantalum capacitors from what I've seen, the grey/white stripe on the end of the package indicates the POSITIVE terminal of the capacitor.

In the metal "can" style capacitors, and the usual plastic/polymer round cylinder style packages the marking is usually for the NEGATIVE terminal. The tall round cylinder packages often have a grey-ish stripe, with obvious black minus signs going down it. On the metal can packages, the negative terminal is usually indicated by a solid black semi-circle/area of the circle closest to the pin, on the top of the case.

This is from experience, using products with more useful datasheets that specifically mention the polarity markings - and then it's just an industry standard that you'll see throughout your time designing stuff.