How can I be sure of sanitation of a restarter before I pitch it

[James David Dexter]

My fermentation has been stuck a couple days so I wanted to give it a little boost. I have made a restarter using some of the wart, a heap of boiled sugar water and some extra yeast. It was getting along fine, actually just continuing to ferment without any sign of krausen so I think the yeast in the wort were actually still active enough anyway, but to give it a little boost I added in a multivitamin tablet. The tablet came from a multipack of multivitamin tablets that I found on my communal houses coffee table and so now I am un-sure as to the sterility of the extra pitch. I wanted to add it anyway to boost the ABV so it did initially have a very high SG. Is there a way to know if this is going to contaminate the rest of my batch before I add it back in, or can I do anything to this to be sure its clean, if at the expense of the yeast colony in it, seeing as the yeast in the main batch will be active enough to finish the extra sugars? Long post I know, cheers in advance