Bree finds out about Lucy and Evan in Tell Me Lies when she stumbles on incriminating photos of the two of them together on Evan’s devices.

Quick Scoop: What Actually Happens

In the college‑era (2009) timeline, Bree is using Evan’s laptop when she comes across a photo of Evan and Lucy at a party the night they slept together. The picture is intimate enough and tied to that specific night, so Bree immediately connects it to the cheating she already suspected but didn’t fully understand.

Earlier coverage also notes that Bree uncovers the secret through photos of Lucy and Evan on his Facebook, which are compromising enough to expose that something more than friendship happened between them. Those images are what finally blow Lucy and Evan’s secret wide open for Bree.

How Bree Reacts

Once Bree sees the photo on Evan’s laptop, she realizes Lucy is the person Evan slept with. Instead of confronting Evan right away, she panics and runs to Lucy’s dorm to talk to her, but Lucy isn’t there, so the confrontation is delayed and hangs over their friendship.

In the later wedding‑timeline, Stephen tries to use the Lucy/Evan hookup as a bombshell reveal, but it fizzles because Bree has actually known about it for years by that point. She and Evan eventually work through it enough to stay together, and Bree keeps the truth to herself, not telling the wider friend group.

So, to answer the core question “how did Bree find out about Lucy and Evan?” :
She discovers their secret by finding incriminating photos of them together on Evan’s Facebook and then a key party photo on his laptop, which confirms for her that Lucy is the girl he cheated with.

Information gathered from public forums or data available on the internet and portrayed here.