If your fern has fronds which are divided three or more times then you most probably have one of the Dryopteris species known as the Buckler Ferns. There are however one or two plants with which they can be confused. In particular it is often the case that gardens are invaded by Bracken (Pteridium aqualinium) from wild stocks. Bracken has several features which distinguish it from most other ferns. Firstly it is generally very large, 4 feet (120cm.) or more. Secondly the fronds rise individually from a dense mat of roots and stems on, and under, the ground, it never forms circular “crowns” with the fronds radiating outwards from a centre. The fronds of Bracken also have side branches that have the very obvious appearance of being like bird’s wings when spread as on landing, this sounds strange but is easy to see in practise. However you may be really certain you do not have Bracken. If you find on the back of your fronds, in summer, rounded, kidney shaped, spore cases or small tufts of sporing bristles (picture 1). Then you may be sure you are not looking at Bracken. This is because the sporing bodies of Bracken which are not common, run along the very edges of the leaves, and are always covered by the rolled over margins of the leaves (picture 2).
Now if you have a buckler fern and it is a large dark green plant, and is in an already established garden. Then it is almost certain to be Dryopteris dilatata, or the Broad Buckler Fern. (Picture and More) If you have a new planted garden however, then you may have one of several foreign species, often with coloured fronds. For these you will need to resort to the Dryopteris section of a reference book.