How can I tell actual rental rates?

no. but be smart and assume asking rent is what is being paid--because usually it is.

In very few cases, the actual rental price is recorded by the broker. On SE this shows up as a price change on the day the listing is removed. I've also seen this opportunity used by shady brokers to artificially jack up the price: e.g., the apt's been sitting for months, they drop the price, a month later it's taken, but the price is jacked up on the day the listing is removed.

More generally, if the building is large, I look for at past listings of similar apts with the idea being that the lowest-listed ones are most representative of where the market actually lives. FWIW, at the higher end (as of about a year ago) I had no problem negotiating more-than-10%-off in a number of places I had interest. This was on already well-priced apts.

Well, in aggregate anyway Jonathan Miller's reports have city-wide (and neighborhood) average rents nominally and after concessions. So you can at least get an idea of the typical discount.

Some of the larger rental buildings appear to be experts at obfuscation, vis a vis, their rental prices. I've seen over and over again, apartments of 1300+ sq. ft. with prices commensurate and way beyond, show up in rented column (and in available listings, as well) as 1 bedroom 1 bath. I've actually seen seen some of these apartments. They are 2 bedrooms, 2.5 baths. The mendacity and obfuscation is mind blowing. Shouldn't be.
I have never bothered to notify SE of this, but I have seen it done continually on at least two rental buildings (different management companies) that I track. So, there are no real numbers, it's like the CPI. Lies and lies. One bedrooms going for $6400, 1300 sq ft. Bullshit. They are taking lessons in fraud from Wall Street and the government.

Thanks, all.

gaongaon - Sadly, "experts at obfuscation" is indeed an accurate descritption of the entire real estate industry!