Check out each product description for the sizing fit within the product description. This will give you an idea on if that particular item runs small, large or true to size. Keep in mind fabric type and where it was manufactured also plays a roll in the fit.
Most products also have a measurement chart within the photo carousel. When in doubt, match up the product's measurement chart with your best fitting top/dress/skirt/etc. and pair it to the measurement/size that most accurately matches your piece.
Still not sure? Email firstname.lastname@example.org and I can help you out.