There is a lot of room for error in making that call, but generally one may go by their costuming. Steampunk music can be identified on a five point scale. Depending on how many of the criterion the band fills, the more Steampunk it is. Bands on the list below should fill at least 2-3.

1. They call themselves Steampunk.
2. Old fashioned dress or style.
3. Classical instrumentation elements.
4. Fictional (preferably alternate history) back-story.
5. Steampunk (sci-fi/historical) lyrics.

They will be marked (SP) (DP) and (CP) other details will also be added such as non English

Let’s start by defining the genres, feel free to correct me if I’m wrong.

Steampunk is defined as Victorian, basically the start of the airship era, musket, cannon and sword. Single ships slugging it out for dominance of territory. These bands I feel represent Steampunk the best, Clockwork Dolls - Dramatis Personae, Abeny Park – Airship Pirate, The Cog is Dead.

Dieselpunk is generally from the twenties through the end of WWII. In the time of organized Arial warfare, squadrons of heavily armoured ships pounding it out with machine guns, mortars and heavy cannons, no longer lightly armoured wood ships but dense constructs of steel and iron. The music is jazz and brass. The Bands or songs I think represent Dieselpunk, The Clockwork Dolls – When Banners Fall, Diablo Swing Orchestra.

Cyberpunk is everything that comes after Steam and Diesel, the best description I can think of is outlaw space ships, think movies like blade runner.

Abney Park – (Sp) (Dp)
Amour Obscur – (Sp) (Dp)
Armitage Shanks - The Carny Preacher – (Sp)
AtOmsk – (Sp) non English
Bad September – (Sp) (Dp)
Cabaret Decadance – (Dp)
The Cassettes – (Sp)
Charles Von Vitic – (Sp)
China Steamengine – (Sp) (Dp)
Circus Contraption – (Sp)
The Clockwork Cowards – (Sp)
The Clockwork Dolls – (Sp) (Dp)
The Clockwork Quartet – (Sp) (Dp)
The Cog is Dead – (Sp)
Coppelius – (Sp) non English
CORSET – (Sp)
Creature Feature – (Sp)
Crystal Bright and the Silver Hands – (Sp)
Dark Design, The (UK) – (Sp) (Dp)
Darwin Prophet – (Cp?)
Deadly Nightshade Botanical Society – (Sp) (Dp)
Diablo Swing Orchestra – (Dp)
Die Giftmischer – (Sp) non English
The Dirge Carolers – (Sp)
The Dresden Dolls – (Sp)
Doctor Steel – (Sp) (Dp) (Cp?)
Drachenflug – (Sp) non English
Dr. Carmilla – (Sp)
Edward Asbury – (Sp) (Dp)
Emilie Autumn – (Sp) (Cp?)
Emperor Norton's Stationary Marching Band – (Sp) (Dp)
Evelyn Evelyn – (Sp)
Escape the Clouds – (Sp) (Dp)
The Extraordinary Contraptions – (Sp) (Dp)
Frenchy and the Punk – (Sp)
Ghostfire – (Sp)
The Hellblinki Sextet – (Sp) (Dp)
Insomniac Folklore – (Sp)
Jarmean? – (Dp)
Jill Tracy – (Dp)
Johnny Hollow – (Sp)
The Langer's Ball – (Sp)
Marquis Of Vaudeville – (Sp)
Metropolis – (Sp) (Dp) (Cp)
The Men That Will Not Be Blamed For Nothing (Sp) (Dp)
Professor Elemental – (Sp) (Dp) Gentlemen Rap
Rhubarb Whiskey – (Sp)
Steam Powered Giraffe – (Sp) (Dp)
Sunday Driver – (Sp)
This Way to the Egress – (Sp)
The Violet Tribe – (Sp) (Dp)
Vixy and Tony – (Sp)
Unextraordinary Gentlemen – (Sp) (Dp)
Vagabond Opera – (Sp) (Dp)
Vagabondage – (Sp)
Vermillion Lies – (Sp) (Dp)
Vernian Process – (Sp) (Dp)
Veronique Chevalier – (Sp)
Victor Sierra – (Sp)
Walter Sickert and the Army of Broken Toys – (Sp)
White Ghost Shivers – (Sp) (Dp)
The Widow's Bane – (Sp) (Dp)

Questionables
The Aeronauts Tankus the Henge
Alexandra Hamer
The Two Man Gentleman Band
Alice Moving Under Skies
Vera Mesmer
Cirque Noir
Voltaire
The Harlem James Gang
Abandoned Toys
Ephemeral Mists
Fermata
Kaizers Orchestra – Non English
Life's Deccay – mix non English
Maciste – Non English
The Scarring Party
Synthetic Dream Foundation

Feel free to add to or argue the choices.