what are signs of a blown head gaskett on 1998 Buick Century

Replaced transmission; water assembly hose/pump;new thermostat and all fluid levels checked and filled in 1989 Buick century w/ 100,000+ miles. I have added anti-freeze twice in reservoir in the last month.No signs that there might be a leak. Once I pull off road and wait the needle falls to center of gauge but within minutes of driving it goes right back to hot.

Asked by for the 1998 Buick Century
is this a front wheel drive car with a 3800, 3400 or 3100 v6 engine? If the answer is yes I must ask, after all these repairs and adding antifreeze did you bleed the air from the cooling system? There is a bleeder screw or screws on these engines, that require you to open during the filling process to remove air from the system. If you didn't do this you might have a air lock in the heads that is causing the overheating. With the engine cool open these screws and fill the system until coolent comes from them with out any air bubbles. Close them and run engine until normal temp. let cool and fill overflow tank only repeat until it stays full. Signs of blown head gaskets are oil in the coolent, coolent in the oil (milky looking foam) white smoke from exhaust, large bubbles coming out of radiator with the cap removed as it warms up.