i did that with some rods food and it was not a pleasant smell ...i think you will know if its bad ...you dont have to stick your nose to it, if you can smell its foul when its opened then its probably bad
If it just thawed out for a little while you are fine. If it was in a hot car for a day probably not. Also fish would eat some food that was not fresh in the wild. Of all the things that we do need to worry about in reefkeeping this docent make my list.