You might get away with it - according to this site shelf-life can be anything from 30 to 180 days. (Off cause you don't know how long they were in storage before you brought them). They start to go soft and smelly when they go off.
onions stored in a cool dry place (i.e. strung up in the garage/shed) can last up to 6 months so i imagine if they've been stored in the fridge they should be ok! peel off the top layer as usual and chop away - if they're brown inside the central core, or growing mould in any place then they're off. You can store them in the fridge if you want, skelly, but i don't bother to (waste of beer-cooling space).