A Sitz marker test is where you swallow a capsule that has 24 radio opaque rings (can be seen on x-ray), then usually x-rays are taken on days 1, 3 and 5. If 20% or more of the rings remain at day 5 , it is abnormal. You must NOT take any laxatives or enemas during the test.
I had this test done preoperatively and all 24 rings remained at day 5.
Hope this helps