There are plenty things that Stanley finds out about Blanche:
First, that she had a very bad reputation at Laurel where she kept a room in a hotel where she was staying after leaving Belle Reeve. We know that she ran a form of brothel out of her room in that a lot of men kept keys to her room and she entertained them, presumably sexually, for money.
Second, he knew that she had lost Belle Reeve do to her own misjudgements, and that she was a destitute. There were no rich friends, nor any social merit that would make Blanche seem respectable in society.
Third, he found out about her shameful firing from the school district when it was found out that she was having sex with a high school student while she was teaching. This, left her marked for good.
Hence, what Stanley found out was who was the real Blanche, and how she had made up an image about herself which now he realizes was a made up tale. And this was a great weapon for Stanley to knock her out of the scene.