There are four sets.

1. Shadow of the Tyrant
2. Secrets and Lies
3. Black Hand, Black Heart
4. Awakenings

Shadow of the Tyrant has no set symbol, the other three have a symbol in the lower right hand corner. The symbol is the same for all card types in the set regardless of rarity. Sample Actions are shown below:

Shadow of the Tyrant (action)

Secrets and Lies (action)

Black Hand, Black Heart (action)

Awakenings (action)

Promo (hero)

Also, holdings from the first 3 sets can be tricky. For Shadow of the Tyrant, the artist text is in white. For secrets and lies, they are both in black, with a partial black background. You basically can't see the symbol on Black Hand, Black Heart, but the hook on Secrets and Lies can be seen:

Secrets and Lies (holding)

Black Hand, Black Heart (holding)