How do I know if my first layer is acceptable?

When you go about getting a sufficient first layer, you make sure that the extruded lines are not too flat, but also not too high that it will not adhere to the bed.

You are looking for a “Goldilocks” adjustment, right in the middle. The extruded lines will have a little bump to them. Adjust your z-level to find the sweet-spot. Check the image below and base your first layer off it: