Ad mysteriously disappears
Sure, the landlord could have taken the ad down because he already has enough applicants. But when the ad disappears as soon as you've applied for the apartment, you know that something is fishy.

Spelling errors
Is that even English? If you ask yourself that question while you're reading the response from the landlord you contacted, you should probably be concerned. Of course, the person may just have an issue with grammar and spelling, but too many typos are frequently a sign that you're talking to an imposter.

Long-distance landlords
So it turns out that the landlord who you contacted lives in a different country. They promise to send you the keys, but only if you pay the deposit or the rent first. It's time to get suspicious.

"Safety"
"Could you send me a copy of your ID as a safety measure?" - even if the mysterious landlord offers to send you a copy of his or her own ID, this is by no means proof that the accommodation is legit. Fraudsters often use copies of the IDs of previous victims. Don't send them any personal information about yourself unless you're completely sure you can trust the person you're corresponding with.

Don't rush into it
If it was up to the landlord, everything would go as quickly as possible. When they double-email you and try to pressure you about something, take your time. Think about it first: do they really have a reason to rush you? Probably not.

Tearjerking reasons for money
...and if they have a reason, it's likely made up. Imposters think of the most tearjerking stories to get your money. Maybe they have a sick mother; or your rent is going to enable them to pay for their own apartment. Fraudsters don't shy away from anything. There are times when you have to be a little suspicious and cynical in this cruel world...

Outlandish stories
...Not least because some of these stories are just ridiculous. When everything is (seemingly) already prepared for you to move in, imposters tend to come up with the most outlandish excuses to get more money. Precisely this happened to me: the fraudster tried to make me believe that his aunt already promised the room to someone else. Although I had already paid him the deposit he said that I would only get the room if I sent him more money. That was when I realised that I'd never see my money again.

Contradictions
With outlandish stories, there often come contradictions. Imposters may ask the same questions twice or state something that doesn't tally with something they said before. In my case, for example, the fraudster claimed that there were two rooms to rent in the beginning. However, when I asked him about the second room later, he didn't seem to remember it. Watch out for inconsistencies in their stories!

Finally... trust your gut
I know, easier said than done. But if it looks too good to be true, it probably is. Don't get caught up in the excuses and lies the imposter feeds you. If you're not sure the person you're corresponding with is a legit landlord, arrange a meeting, preferably in the apartment in question. Even a video or phone call can expose a fraudster. It's a positive thing to see the good in people, but sometimes, it's better to be careful, especially where money is involved.