How do you know? Do you mean there are none visible?
On some vehicles, it's hard to tell if you need a head gasket until the head is off.
My 1998 Honda Civic, which I still drive, had a similar problem. It would occasionally run a little hot when I got off the highway (and my air conditioning was broken). Eventually, after ruling everything else out, I bit the bullet and had the head gasket job done, and it hasn't overheated since.
Think of it this way: Whether the head gasket was the cause or not, the brief periods when the engine was hotter than usual very likely warped the head, even if it's just by a very small fraction of an inch. Whether the head gasket was the original cause or not, you likely need a new head gasket now, which will likely include planing the head(s).