I have been given some kefir grains but, no having tasted kefir before, I can't tell if there's something wrong with it.
Is there some way to tell?
For the record, the taste is like a mix between yoghourt, cheese and sour milk.
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If the taste is similar to yogurt, cheese, or sour milk as you describe then your kefir is likely good.
On the other hand, if it seems moldy or rancid, it is bad.
This site describes proper storage, shelf life and how to tell if it is bad.
And this site describes kefir gone bad if "f there is noticeable fuzzy mold growing on it, if there are pink or orange spots on it, or if it has turned pink or orange. Your body knows when it is bad, it should literally be revolting to you if you attempt to ingest it."