Read article and there are formulas at the bottom for test solutions.
I can't find a formula for the stuff that Kodak or Ilford sells but the formula's I do know of for mixing Selenium toner from scratch don't contain ammonium thiosulfate. The "Rapid Selenium" toner formulas may however contain ammonium thiosulfate (not sure why because residual fixer is what causes the ugly staining all the instructions warn you about) and I'm pretty sure they contain ammonium thiocyanate (as a silver reducer / bleach?). You could use a low dilution of one of the prepared selenium toners as a residual hypo test, looking for stain. But I have no feel for its sensitivity.
And yes, there is a distinction for testing for residual hypo and complete fixing. Testing for residual hypo is just that, you are looking for fixer (hypo) left behind in the paper. Testing for complete fixing, you are looking to see that all the unreacted silver has been removed.
"hypo" is a nickname for fixer, left over from when sodium thiosulfate was the primary photographic fixer and was also known as sodium hyposulfate. Rapid fixer formulas rely on ammonium thiosulfate which acts more quickly and has a higher working capacity.