I don't think there are many normal sentences containing the sequence יש לא. More generally, it won't be easy to find two sentences that make sense and differ only in לו vs. לא, so context would help. But... if you're a learner, you won't always understand the whole context either.
Not sure, but I think most often לא would be more stressed then words around it, and לו would be less stressed. As a learner, I'm not sure you can hear that, but maybe.