Ifyou saw the blossom, then its easier to tell - the blossom on bird cherry (not poisonous, but unpleasant to eat Prunus padus) looks like little candles all over the tree, the blossom stands upright. With most other cherries, the blossom doesn't do that.

You won't be poisoned by tasting some. If they're about 1cm across, and yellow/red, they will be ok to eat, but may be a bit sharp. Dark red, and they're probably eaters, but, beware, Morello cherries are dark and they are not too good raw (that's an understatement!), needing a lot of sweetening. If they're edible, try them in a pie, and win one over on the blackbirds.

Size as well my P. avium and P.padus are big trees wheras Stella for example is about like P. kanzan the ornamental double pink blossom tree

I would think it impossible to eat enough bird cherries to come to any harm. I once bit into one and spat it out immediately, Ugh!

As far as I know, non of the cherry (prunus) species are poisonous. However, many of them, especially the fruit of ornamental type, don't make good eating

Be careful though, because the name cherry covers a number of species, being a particular subfamily of the prunus, i.e. winter cherry, aka Christmas Cherry (Solanum pseudocapsicum) which is poisonous.

If like you you said, it's a tree, then this will rule out the poisonous types and I'm sure that it will be edible.

Taste one and if it tastes of cherries you're in! If it tastes horrible spit it out and no harm done. Morellos are naturally less sweet but not horrible, and do well in a north facing situation.