by checking with some one who really knows their wild berries. in the past you learnt from older family members what to eat and what not to. now because food from plant to plate is done for us many people don't know.

if in doubt don't eat it. just because birds can eat them doesn't mean we can.

About the only way really is to identify accurately the plant they're growing on, by the growth habit, leaves, stems, foliage, flowers and yes, whatever fruits it bears, and that's not always an easy thing to do

Suck it and see! No, don't. Get a good book on the subject or, better still, a friend who knows about these things.

Or put a photo on here, with plant details, remembering Bamboo's advice.. Somebody will know.

The same way humanity has been finding these things out all along...ask those who lived to give you an answer. Books can be good or bad. For instance, there are mushrooms which look exactly alike yet one can kill the liver of the consuming it yet another is harmless. It’s silly to get sick for a little bit of taste.

As a general, but not universal rule, berries are unpleasant to the taste when they are bad for you. I've tasted (and spat out) quite a number of poisonous berries in my time and can confirm that you'd have to have no tastebuds to consume enough of them to really get sick.

As had been said, find out before you taste something. If you found an unlabelled bottle of yellow liquid you might think that could be chardonnay or bleach. You wouldn't think I'll have a good swig and find out.

Loosestrife2's point about mushrooms is a good one. It tends to be geographically related. That is, a mushroom in China is edible but one that looks exactly like it in Australia is toxic. I use that example because a Chinese chef died a few years ago when on a visit to Australia because he didn't understand that point.