1994 C4 Corvette Overheating

My aunt bought a 1994 Corvette with the 5.0 OHV
motor. On the drive home it started overheating, she
bought a thermostat and had her mechanic put it. He
pressure tested and everything showed fine, he said
the lines flowed freely. It has been running well for
awhile now but today she took it out and went about
40 miles round trip, and the gauge almost went to the
hottest line. She stopped turned it off but didn't hear
any noises or any other tell tale signs of overheating
that she has seen experienced before. Any tips of
what to try next if we dont have a temp gun? Thanks.