Check the ACA International website to see if they are a member of the Organization. Although the fact that they are not a member does NOT mean they are a scam. You can feel pretty confident making payment to a company that is a member if they can prove to you that you really owe the debt.
Ask for the Company's Website
Most reputable debt collectors should have a website that looks presentable. Although they don't have a website, it doesn't automatically mean they are a scam company, it certainly raises red flags.
Google the phone number the company called you from.
Make sure there are not a significant amount of allegations that the company is harassing or abusing consumers.
Ask the collector how they receive payment.
If they ask you to Western Union the money, for example, you should have cause for concern.
Are they overly Aggressive?
If so, assuming the other clues are met, chances are they are a scam collector. Many of them operate offshore and could care less about American law and American Legal Penalties. As such, they are much quicker to cross the line, and have total disregard for the applicable law (FDCPA).
Discern if the caller is calling from overseas.
I want to tread carefully here. While being cognizant of political correctness, I also want to provide you with the most accurate information available. With that said, 99% of the illegitimate debt collectors that I have come across speak in very heavy accents yet have very americanized names. Just because a debt collector doesn't sound exactly like you or me does not mean that they are a scam. However, if the collectors have very thick accents and something just seems off, assuming the other flags above are met, I'd almost be positive that it is a scam debt collector that is contacting you.
When in doubt do not pay
Now, let me clarify here. If you know you owe a debt, I'm not saying not to pay it. I'm rather suggesting that you make sure the company you pay it to has the rights to collect the debt. I've heard dozens of dozens of consumers who made payment to a "debt collector" only to find out the collector was not collecting on behalf of the original creditor and just took the money and "ran". If this is the case, you will still be responsible for the entire debt you owe to the original creditor.
If the collector is legitimate, they will do whatever it takes to have you pay them. As such, feel free to ask for proof that the debt is owed, that they have authority to collect the debt, and that any payment made will offset the balance currently owed.