Check there is no sign of moisture inside the glass then put it in a tank or bucket of water, not plugged in, and leave it a couple of days. Check to make sure there is still no moisture inside.
If there is moisture - throw it away. In none, it is probably safe to use. However for added safety you should have an RCD on the power outlet feeding the heater so that if there is any chance of electrocution or shock, the RCD will switch off the power.
Leave it running a few days to check it stabilises and holds the correct temperature. Old heaterstats tend to stick ON, not OFF.