Ahsoka is often seen accompanying Anakin interacting with Padmé, so it's evident that she would know of it. She also reaffirmed this during the scene she left the Jedi Order.

Her eyes.

Seriously though, she was his padawan, and strong in the force. Obi-Wan knew, but chose to ignore it, Ahsoka did the same. They didn't really hide it that well. I don't think everyone knew the extent of their relationship, marriage and pregnancy, but their feelings weren't difficult to miss.

She’s a force user.

She senses it too.

Because it was kind of obvious considering how often she was around Anakin and Padmé and could tell they cared for each other at a more than “friendship” level.

Somebody sent her a text pointing her to a “leaked discount store footage” web site where they were seen making out rather crassly and buying baby diapers.

Possibly she pieced it together like Obi-Wan was able to since Ahsoka was around Padme a lot so she could tell there was something going on with them

Because she’s not stupid she spends a lot of time around Anakin, so it makes sense she would see right through it

Plus if I am not mistaken if Order 66 didn’t happen instead of Sabine it would be Leia who would be Ahoska’s apprentice while Yoda and Obi Wan argue over who gets Luke