I have collected many many pen tests.
Usually they wrote alphabets, or scribbled familiar words. Sometimes there are doodles, but most of them look like they were drawn by someone who simply liked to draw or sometimes they are recognizable as children's scribbles. Sometimes it looks like someone was learning to write the alphabet (repeating many of the letters).
This kind of pen test is common:
Sometimes the folio is covered in letters and drawings:
Usually they wrote alphabets, or scribbled familiar words. Sometimes there are doodles, but most of them look like they were drawn by someone who simply liked to draw or sometimes they are recognizable as children's scribbles. Sometimes it looks like someone was learning to write the alphabet (repeating many of the letters).
This kind of pen test is common:
Sometimes the folio is covered in letters and drawings: