how do I know whether my ford explorer 1999 engine has a thermostat or it's been removed with out checking it inside its housing .or it must I disassemble the housing and inspect it physically?

Use an infrared temp sensor and monitor the temp at the inlet to the radiator from a cold start. When the engine temp rises, this location should remain fairly cool until the thermostat opens and allows hot coolant to flow into the radiator. That will cause your temp reading to jump quickly (usually between 1980F and 195F). If it does not, but just continues to rise slowly into the 200F plus range, you probably have a failing or missing thermostat.

In the U.S. coolant temperature coolant is NOT clear. Suspect the previous owner used only water in the radiator, which is not good. I have a 99 explorer V6 OHV engine. The thermostat housing only has two bolts and is on the top front of the engine. May be different there.

If your temperature gauge does not work, the problem can be in the temp sensor. It should be located in the vicinity of the thermostat housing. Make sure it is plugged in. They are not expensive here in the U.S. (a couple of bucks) and screw in.