How do I test tension on a scrap?

Use a doubled scrap of the actual fabric, contrasting thread top and bobbin, and judge a straight stitch from both sides. Every part of that sentence is doing work.

The actual fabric. A tension right for calico is not right for the silk you are about to sew. A test on the wrong cloth tells you nothing useful.

Doubled, because that is what a seam is. A single layer behaves differently and will mislead you.

Contrasting thread, top in one colour and bobbin in another. On matching thread an unbalanced stitch is invisible until the garment is finished and the seam fails.

A straight stitch, because the geometry of a zigzag is not symmetrical and cannot be judged the same way.

What you are looking for. The two threads should lock inside the fabric, so neither colour shows on the opposite face. Bobbin colour visible on top means the top tension is too tight. Top colour visible underneath means it is too loose, or not seated in the discs.

Sew a decent length, at least 10 cm, at the speed you intend to work at. Some tension faults appear only once the machine is moving properly.

Then test the seam itself. Pull it gently across the stitching. A balanced seam resists evenly; a loose one gives and shows thread.

Keep the scraps. Write the settings on them in pen and keep them with the project. It costs nothing and it saves repeating the test.