Who determines whether a business activity is ethical?

your conscience

as long as it feels right and not too evil

also if you're really stuck then just ask a REALLY GOOD person that knows whether the business activity is ethical

example : your family, a friend, your boss if he's not evil or just ask a random stranger (p.s. they must be good at knowing what is the right thing to do also)

if you're asking this question to do some business activity requested by your boss that you are unsure of whether it is ethical or not, then in doubt, ask your boss (you know unless he's evil), then ask your boss' boss if you have any

but if your boss requested that you DO so even if it seems unethical, well then that's up to you

Note : doing unethical stuff is NOT illegal but it may and most likely make you feel bad unless you're evil anyway