There are a few tips I've discovered during years of hunting for different apartments to weed out suspicious listings. Your situation does seem reminiscent of a time I was nearly scammed on a rental property where the listing was a guy who had moved(from Michigan) to New York for a job and still owned his house in Michigan. We really liked the pictures of the property, but were unable to actual tour the home because the "owner" was in New York. He would send the keys by mail if we agreed to rent. Long story short, we came across a new opportunity and moved to North Carolina instead. However, I've learned a few things since that time that may be useful.
First, make sure to Google the address of the rental property and carefully scan the results. Typically when a person is scamming rental properties, the home is up for rent on some other site or the pictures are readily available on a site like Zillow or any of the other many rental listing websites. If you see it on another site with a different price or different details, there's a good chance you're being scammed.
The thing to look at with the pictures is to first confirm the interior pictures match with the Zillow listing. Additionally, it's wise to compare the pictures the potential renter sent you against these pictures online. Are there any pictures sent to you that are not on the listing on Zillow? Are there any that don't seem to match or correspond to what you see on Zillow?
A site like Zillow can be VERY useful in your search. Not only does it list tons of information about the properties you're looking at, but it also shows other legitimate listings to compare against and choose from. With the other properties in the same neighborhood, you can compare the price and see if the property is worth renting simply by using it's comparative value. If you're looking at the cheapest option in the neighborhood, there's a good chance there's some issue not being shown. If it's about average, you're probably okay.
You can look at sales data to see when the property was last sold and usually get an idea of who it was sold to. While not always the most useful, you can see if it sold to a property management company or a private buyer and you can see the appraised value at the time of sale. This can tell you whether the property is worth the monthly cost and if the property has been updated(value has gone up). Additionally, if you're seeing the property sold fairly regularly, there's a good chance it is a rental.
On top of all this, it may be wise to carefully scan the lease agreement and compare to other lease agreements you've signed. Compare to your current lease and make sure that it looks legitimate. Scammers will tend to make mistakes and it can become very clear that the agreement was written by someone who doesn't really know what they're doing. This can be helpful combined with other details, but is not always indicative of a scam. If it really is just a guy subleasing his apartment, he could've written it himself.
