How can I check soil moisture to decide when to irrigate without a meter?

You do not need an expensive instrument to judge when to water. The feel method is reliable and free: take a sample of soil from the root zone depth, not just the surface, squeeze it in your hand, and judge it. If it is dusty and will not hold together, it is dry and needs water. If it forms a ball that holds and feels moist, there is still enough water and you can wait. If water comes out when you squeeze, it is too wet and you should not water. Check at the depth where most of the roots are, because the top can look dry while the root zone is still moist. Do this at a couple of spots across the field since soil varies. Over time you will learn how your soil feels at the point where the crop should be watered. If you do want a tool, a simple tensiometer is cheaper than soil tests over a season. For your crop, ask your KVK what root zone depth to sample.