$1800 per month isn't unheard of in the city these days, so many of these places may be real listings. Getting a real estate agent is a great way to make sure you're only looking at real listings, but it isn't the only option.
If you find a listing that you're unsure of, search for the address on the internet and see if you can find the listing on multiple websites. A real listing will likely show up in several places. If there is every a listing that asks you to send money or sign a contract without first seeing the property, don't! That's a good sign that a scam is involved. Calling whoever is listing the property and asking them for a showing is going to be the most sure-fire way to determine if it's a scam. If you are offered a chance to see the property and the application process for getting the apartment is normal (credit check, application form, deposit via cashiers check etc), you're probably looking at a real listing.
If you'd like help with any specific listings, don't hesitate to contact me, and I'd be happy to assist.