The single most important check
Never trust a license number or "regulated by..." badge shown on a broker's own website - it's trivial to display any number or logo. Go directly to the regulator's own public register (ASIC Connect, the FCA Financial Services Register, CySEC's register, etc.) and search independently.
What "fake" actually looks like
A few common patterns: a license number that doesn't exist on the register at all; a real license number belonging to a completely different, unrelated company; a license that's expired, suspended, or revoked but still advertised as active; or a license that's real but for an entity other than the one actually opening your account.
Entity matters more than brand
Large broker brands often operate multiple legal entities across different jurisdictions, each with its own license (or lack of one). The brand name being regulated somewhere doesn't mean the specific entity you're signing up with is - always confirm which entity is on your account opening documents, and check that specific entity's license.
Cross-check with published warning lists
Many regulators publish their own lists of firms found to be operating without authorization or impersonating licensed firms. Searching a broker's name and domain against these lists, in addition to the main register, adds another layer of verification.