You can't always tell, but if they smell bad they probably are. But some people have a lot more sensitive sense of smell than others. Of course a biologist can try to culture bacteria from it and see...
If the meat does not fit in the shell, if the shells are spotted with black or the meat appears discolored. This does not include tiger shrimp, which naturally have a dark shell with black stripes....
As raw beef starts to turn bad it will begin to discolor, you will see various shades of pastle green and yellow, as it gets worse the discolored areas will begin to get slimey and you will notice a...