While there is no way to guarantee that you are dealing with a real landlord/realtor, if the dwelling really exists or if the person you are emailing is authorized to rent it, there are "red flags" to look for in a fake "dwelling for rent post" or emails.
1) Low rental price for area, did not hire a management company or google map can't find address
2) Owner out of town/state/providence/country doing charitable works
3) Owner says the keys will be shipped to you or won't give you a phone number
4) Owner mentions religion, god-fearing or cleanliness repeatedly
5) Owner sends you questions as the "application" in the email instead of using an attachment
6) Owner says you are the perfect renter but you can not view the dwelling for any reason
7) Owner uses incorrect terms for the area such as hob/stove, water/hydro, gas/petro, color/colour, trash/rubbish/garbage
7) Owner misspells common words, uses strange punctuation or Odd Capitalization
8) Owner says all utilities and monthly fees are included
9) Owner says you have to fill out a credit application to get your credit score and includes a link to some site
10) Refuses to answer or answers vaguely your questions about: closest public transportation and what type (train/taxi/bus/subway/tube/etc.), name/location of electric/gas/garbage/water companies, closest medical facility/library/grocery/bank/dry cleaners/post office/McDonalds
And the BIGGEST red flag of all:
Owner wants first/last/deposit via Western Union or moneygram
( or suggests you send money to a friend via WU or MG as proof you have the funds )
Any one from the list is highly suspicious, 2 or more and you definitely have a scammer. No. Exceptions. EVER. For any reason.
Western Union and moneygram do not verify anything on the form the sender fills out, not the name, not the street address, not the country, not even the gender of the receiver, it all means absolutely nothing. The clerk will not bother to check ID and will simply hand off your cash to whomever walks in the door with the MTCN# and question/answer. Neither company will tell the sender who picked up the cash, at what store location or even in what country your money walked out the door. Neither company has any kind of refund policy, money sent is money gone forever.
Now that you have responded to a scammer, you are on his 'potential sucker' list, he will try again to separate you from your cash. He will send you more emails from his other free email addresses using another of his fake names with all kinds of stories of great places to rent, lottery winnings, millions in the bank and desperate, lonely, sexy singles. He will sell your email address to all his scamming buddies who will also send you dozens of fake emails all with the exact same goal, you sending them your cash via Western Union or moneygram.
You could post up the email address and the emails themselves that the scammer is using, it will help make your post more googlable for other suspicious potential victims to find when looking for information.
Do you know how to check the header of a received email? If not, you could google for information. Being able to read the header to determine the geographic location an email originated from will help you weed out the most obvious scams and scammers. Then delete and block that scammer. Don't bother to tell him that you know he is a scammer, it isn't worth your effort. He has one job in life, convincing victims to send him their hard-earned cash.
Whenever suspicious or just plain curious, google everything, website addresses, names used, companies mentioned, phone numbers given, all email addresses, even sentences from the emails as you might be unpleasantly surprised at what you find already posted online. You can also post/ask here and every scam-warner-anti-fraud-busting site you can find before taking a chance and losing money to a scammer.
If you google "fake apartment rental", "fraud Western Union house rent scam", "fake craigslist apartment rent scam" or something similar you will find hundreds of posts from victims and near-victims of this type of scam.