Honestly I struggle too, I have clothes from when I was at my skinniest and I use those are guides. My mum actually bought me a very fitted dress for xmas and its a size smaller than I have been wearing last year. Luckily it fits but as it is so fitted I would feel far better if I was say 10 pounds lighter.
I really struggle with weighing myself, a few years ago it becamed a problem and I became very disordered with my eating and exercising habits. I looked great but I wasnt healthy at all. So i tend to just go by old clothes, every month i will try on a pair of my skinny shorts or whatever and see how they fit comapred to the month before.