This is a simple extrusion test I mostly use to dial in my extrusion multiplier.
Instructions:

• Print WITH supports! (WHY? So that first layer inconsistencies don't ruin the results)
• 100% rectilinear infill
• 1 perimeter
  I usually print them in 5% intervalls.
  After printing judge the top-layer:
• gaps: flow too low
• nice and flat surface: flow good
• rough surface where the nozzle plowed though: flow too high