You develop a sense over time. It can't really be quantified in my opinion. You can quantify interest, but women often fake interest to be nice and avoid a confrontation, and the bigger issue is that just because she is actually interested doesn't mean she will go on a date with you. She may have a boyfriend, may not be in the right head-space, may want to date but deliberately denies herself due to job or school or god only knows what, or she may be crazy. Bottom line, you can't tell.
It sounds like you're trying to avoid rejection. Get over that. You say you don't want to waste time but how much time are you wasting sending her that initial text? I know it's disappointing to not get a response back, but you have to get over that. It happens to the best of us. Frequently. And besides, it's not just the first text girls flake on. They will flake on you after your first date. Second date. Second month. Etc. You just have to keep on keeping on.
Also don't ever send a girl two texts with no response from her. Don't delete her number either. Send your text and forget about her unless you hear back. Go in with low expectations. Get more numbers - that way you won't be hit so hard by the flakes.