You want a method of testing the soil pH. Test strips are just for liquids. If money's a problem, call around and ask how much the capsule-type soil test kit is at nearby garden centers. Some places sell them for as little as $5. Regardless of what's causing the damage, you need to check and correct the soil pH first or at the same time as feeding.
I'm sorry to be the one to say it, but if you can't afford a soil pH tester and/or sources of Nitrogen, Phosphorus and Potassium, then you can't afford to grow right now.
I'm going to amend my previous diagnosis and say the plants probably need some Potassium as well. It's only partially visible in the pic, but that worst leaf is showing downward & inward curling with damage to the leaf blade edges that is all common with K def. As has been mentioned, the pH can be the ultimate cause of these deficiencies but the plants need those nutrients either way.