What this video covers
The clip covers cheap first-pass checks you can run before paying for anything.
The bleach test is the surface one: dab a small amount on the mark and watch. If it lightens quickly, you are probably looking at mold or mildew rather than dirt or a stain.
The takeaway
These checks tell you whether something is biological, not what species it is or how far it has spread. Use them to decide whether to escalate, not as a final answer.
Read the full written guide: Mold Testing Guide.