These are one of the easiest staphylinid subfamilies to recognise, principally on account of their very large and prominent eyes. The 75 British and Irish species are divided into two genera: 74 species of Stenus, plus Dianous coerulescens which you’ll probably only find by specifically searching beside waterfalls and fast water.
The RES Handbook on Staphylinidae by Lott and Anderson (2011) includes new keys to Steninae by Roy Anderson: buy it here.
I have found the genitalia illustrations of Steninae by Wüsthoff (1934) to be a great help, and even after the publication of Roy’s new keys, it will continue to be useful, if only to those on the look-out for species new to Britain. With Roy’s help, I have updated Wüsthoff’s nomenclature and a 7MB PDF file is available online or email me for a copy.
Note that most of the comments below are about problems with the test keys which Roy Anderson issued before the RES Handbook was finalised. They are retained here for historical interest only. Comments relating to the keys as published in the RES Handbook start here.
Lott, D. and Anderson, R. (2011). The Staphylinidae (rove beetles) of Britain and Ireland, Parts 7 & 8: Oxyporinae, Steninae, Euaesthetinae, Pseudopsinae, Paederinae, Staphylininae. Handbooks for the identification of British insects, volume 12, part 7 [sic]. St Albans: Royal Entomological Society.
Wüsthoff, W. von (1934). Beitrag zur Kenntnis der mitteleuropäischen Stenusarten. Entomologische Blätter für Biologie und Systematik der Käfer, 30, 62 – 64 + 4 plates (“Tafel I-IV”) between pp. 64 and 65.