How can I tell whether a skill is a "default weapon attack"?

The type of skill you're looking for is called a Default Attack Replacer (DAR). You can tell if a skill is a DAR by checking its description tooltip - if it's a DAR, it will include the phrase "When used as your default weapon attack..."

The full list of DARs in the game (not including your character's default attack) is:

• Cadence (from Soldier)
• Savagery (from Shaman)
• Fire Strike (from Demolitionist)
• Righteous Fervor (from Oathkeeper)
• Belgothian's Strikes (from Belgothian’s Slaughter set)
• Touch of Chaos (from Rah’Zin’s Torment set)
• Fleshwarped Strikes (from Heart of Theodin Marcell)
• Runic Bolts (from Runebinder’s Spellthrower)
• Troll Rage (from Mistborn Talisman)
• Beronath’s Fury (from Shard of Beronath)

Only DARs (and default attack) can trigger Weapon Pool Skills (WPS), attack modifiers that proc off of autoattacks. You can tell if a skill is a WPS via its tooltip as well; WPS will include the phrase "X% Chance to be Used."

Note that most procs on items (phrased "X% Chance on Attack") are not WPS, though they look similar. For instance, the Direwolf Claw skill on Mythical Direwolf Crest is a WPS, while the Bear Claw skill on Mythical Sigil of the Bear King is not.

You can read more detail on how these two skill types interact in this guide.