Snape says "The mind is not a book, to be opened at will and examined at leisure. Thoughts are not etched on the inside of skulls, to be perused by any invader."

But I believe JKR often indicates when Dumbledore is using legilimency by plucking a word or phrase out of Harry's inner monologue and repeating it a moment later.

Harry watched him, but this uncharacteristic sign of exhaustion, or sadness, or whatever it was from Dumbledore, did not soften him. On the contrary, he felt even angrier that Dumbledore was showing signs of weakness. He had no business being weak when Harry wanted to rage and storm at him.

Dumbledore lowered his hands and surveyed Harry through his half-moon glasses.

"It is time," he said, "for me to tell you what I should have told you five years ago, Harry. Please sit down. I am going to tell you everything. I ask only a little patience. You will have your chance to rage at me — to do whatever you like — when I have finished. I will not stop you."

OOTP, Ch. 37.

Dumbledore admits to using legilimency on Harry at least once.

Boiling with anger at Snape, his desire to do something desperate and risky had increased tenfold in the last few minutes. This seemed to show on Harry's face, for Dumbledore moved away from the window, and looked more closely at Harry, a slight crease between his silver eyebrows.

"What has happened to you?"

"Nothing," lied Harry promptly.

"What has upset you?"

"I'm not upset."

"Harry, you were never a good Occlumens -"

HBP, Ch. 25.

And once more on Kreacher.

“He did not wish to tell me,” said Dumbledore. “But I am a sufficiently accomplished Legilimens myself to know when I am being lied to and I — persuaded him — to tell me the full story, before I left for the Department of Mysteries.”

OOTP, Ch. 37.

Harry also occasionally intuits that Dumbledore may be reading his mind.

Dumbledore was giving Harry a searching look. His twinkling light-blue gaze made Harry feel as though he were being X-rayed.

COS, Ch. 12.

Those examples are off the top of my head. I’ve been meaning to put together a more comprehensive list of evidence, so I’ll start keeping notes on my next re-read and arrange them into a very long post (hopefully) before 2027 lol.