“I’ll Suss It Out” Means “I’ll Work It Out”

In casual Australian English, suss something out means to understand a situation by looking, asking or thinking it through. It does not simply mean to suspect something.

A: Which form do we need for the booking?
B: I’m not sure. I’ll suss it out and get back to you.

Here, the speaker means they will find out and reply. A pronoun normally goes in the middle: suss it out, not suss out it. The expression can also refer to understanding a person or weighing up an option, and its tone is distinctly informal. It is common in British English as well, so it is not uniquely Australian. In a formal email, I’ll check or I’ll investigate is clearer.

https://dictionary.cambridge.org/dictionary/english/suss-out
https://www.abc.net.au/listen/programs/brisbane-mornings/how-to-avoid-online-scams/13293788