The thermostat should have a little tiny arrow stamped on it somewhere to indicate which way the coolant flows. But to help out, the plug shaped part of the stat usually faces the engine block, where it is subjected to the coolant flow, as this plug is heated, the wax inside expands opening the stat up to allow coolant to flow through the hose(s).
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The thermostat should have a little tiny arrow stamped on it somewhere to indicate which way the coolant flows. But to help out, the plug shaped part of the stat usually faces the engine block, where it is subjected to the coolant flow, as this plug is heated, the wax inside expands opening the stat up to allow coolant to flow through the hose(s).