The simple way to establish if an NPC is evil is to just go ahead and attack them. They probably deserved it anyway. If they were good guys, you wouldn't be able to kill them so easily, after all.

Force Drain, by the way, is actually a power in the Star Wars Expanded Universe, a power that we never get to see in the movies. This Force Drain in the comic is not that Force Drain, because of course neither Star Wars nor the Expanded Universe exist in the comic.

Transcript

{Count Dookû arrives at a cave set in the side of a cliff. Obi-Wan and Anakin arrive hot on his heels and race inside after him.}
[SFX]: vzooom!
Count Dookû: Ah am glad you 'ave survahved ze arena. Listen to moi. Palpatine, 'e is ze foe! We must—
Anakin: Don't listen to his poisonous lies!
Count Dookû: Cannot we discuss zis matter lahk civilised zhentlemen? Let me show you ze Peace Moon plans—
Anakin: Never!
{Anakin charges}
Obi-Wan: Anakin! No!
{Dookû uses a lightning ability to stop Anakin}
[SFX]: KRZZRKZKZZZZT!
Count Dookû: 'ow dare you attack moi!
Anakin: See, he's clearly evil!
[SFX]: zzt... zzzt {Anakin is incapacitated, sparks still crawling over his body}
GM: Actually, Force Drain is a defensive technique. It feeds off the attacker's anger.
Obi-Wan: Sorry about my Padawan. He gets a bit enthusiastic sometimes.
Count Dookû: Ah understand. Mah son is sometimes ze same way.
Count Dookû: But what can you do? Zey are young. Zey zink always zey are right.
Anakin: Stinking Sith!
Padmé: But... Annie is always right!