In the beginning of Order of Phoenix, Mrs Figg tells Harry not to put his wand away as "there might be more of them around".
I assumed she could see the Dementors. (They didn't attack her, so the only way she could have known they're around by seeing them. Maybe she felt them, but it's unlikely for a Squib who virtually never left her house to know that feeling too well; and jump to the conclusion that there are Dementors in Little Whinging.)
At the hearing, the Wizengamot aren't sure if Squibs can see Dementors. Harry suspects she has only seen a picture of a Dementor. But she does describe the feeling of having a Dementor around accurately.
So which one is it? Can she or can she not see Dementors?
EDIT: I disagree this is a duplicate of How Mrs. Norris communicates with Filch. That is a different question, as evident from the wording. The connection between squibs and dementors has been mentioned in passing and is not definitively answered there. Please read the answer here and my comment below it.