How do you know if a plate is oven proof?

It will probably say so underneath. If a piece does not have the Words "ovenproof" proceed with caution. Although all pottery is kiln-fired, that does NOT mean that it can safely cope with the (much lower) temperatures in a domestic oven.

(The reason that non-ovenproof ware cracks in an oven is:

(i) expansion takes place too fast; and

(ii) there may be a residual water content that expands into steam.

It follows that almost all ceramic objects can be reheated to high temperatures, provided it is done slowly enough to avoid expansion-cracking).