when did he see her use the force? was it because she heard his call to her in empire strikes back?
shareI always thought that statement was more about telling her that she is his sister and not really a statement about her being strong in the force.
shareLuke saw her wearing the slave girl outfit on Tatooine so he knew she had it, she had it goin' on indeed.
Seriously though, I think he mostly just presumes that being his twin Leia would've most likely inherited the Force like he did, but I quite like the idea that her being able to hear his call at Cloud City was a sign she could use the Force.
The Skywalker family line is naturally strong with the force for reasons never explained, so being his sister, she is as well. And like with Luke prior to meeting Obi-Wan, her ability has been dormant/unrealized but with training she would be able to realize her potential as well.
shareObi Wan told him as a Force ghost on Dagobah that she was his sister (he figured it out when OW mentioned his 'sister' - what with Leia being the only female in the galaxy apart from Mon Mothma and some of the staff on Hoth), and I assume he assumed Obi Wan was assuming he would assume Obi Wan was implying it was about the Force, not just about being siblings
what I'm not actually sure of, is how Luke knew to call out to Leia when he was dangling underneath Cloud city - given that at that time he had no idea they were related or connected. I assume it's just.....Force stuff