>How do you know which berries are edible?
The safest way is to walk with someone that knows and ask them to help you. If you regularly walk the same footpaths during each season you get to recognise the trees and shrubs in leaf
and berry so identification is easier.
Blackberries, rosehip and hawthorn are pretty easy to identify from pictures and reference guides. You can get rosehips from cultivated roses but apparently wild and dog rosehips are better, with more concentrated flavour, so any that you see in a wild hedgerow will be ok.
Sloes can be a bit more problematic if you don't have someone to point them out to you as there are various wild plum types including bullace and damsons which may look similar to the untrained eye (ooh get me!!). Sloes are about blueberry size though and very similar in colour, many will have a greyish blush on them. Check for thorns too as sloes grow on blackthorn. Wild plums grow on thornless trees.
Good luck :)