The only real sign of it is when the murderer/diner worker yells, "I ain't serving him!" in reference to Tibbs.
We don't see any "Coloreds only" restrooms or drinking fountains.
And we can only guess that the hotels are segregated and really bad conditions for blacks when the auto mechanic takes in Tibbs.
Why not show how it really was bad for blacks in the south?
The movie took place in 1966, which was two years after the passage of the 1964 Civil Rights act making segregation illegal. So while discrimination still existed (the hotel in town would tell Tibbs that they didn't have any rooms available), it couldn't be out in the open, like 'Whites Only' signs.
shareIt came out in 1967, being there, we didn't need anything explained to us!
The director probably didn't even think he would have to put in more indicators for future generations. And it's debatable that he would need to. Some things have changed, not much.
. Ephemeron.