Are antique tea cups safe to drink from?

Porcelain itself is fired to a high temperature around 1300 C. Lead as a flux on silica is not really useful at that temperature; it volatilizes out of the glaze melt and goes out with the gases escaping from the kiln.

The place you can find lead however is in the colorful overglaze enamels that are sometimes added in separate lower temperature firings. Those traditional overglaze enamels are a highly lead fluxed composition and are fired so low as to be very unstable for the leaching of lead from the fired surface.

On the flip side, the usual amount of the surface actually covered by overglaze enamels is usually small. So the amount of glaze surface available to leach lead is small.

So if you are talking about a plain porcelain piece, it is perfectly safe. With overglaze on it, if it is a tiny amount painted on the inside on the liquid contact surface, your are probably ok (but no guarantees). If it is a large amount of overglaze inside, take a pass on that one.