WARNING: The following contains spoilers from The Amazing Spider-Man #81, on sale now from Marvel Comics.
During an encounter between Miles Morales and the new Spider-Man in The Amazing Spider-Man #81 (by Saladin Ahmed, Carlos Gomez, Bryan Valenza and VC's Joe Caramagna), Miles quickly deduced that it wasn't Peter Parker behind the mask. In fact, it is Ben Reilly who now occupies the role of Spider-Man while Peter is recovering from a coma.
This is highly impressive for Miles to figure out especially considering the fact that the rest of the Marvel Universe seemingly hasn't caught on. Even villains who have known Spider-Man longer than Miles haven't discerned the truth. Yet Miles was able to figure out it wasn't Peter just by hearing Ben speak for only a few seconds, although this might make sense given how close Miles is to Peter.
The two Spider-Men have a special bond. They share a name, something Peter was reluctant to do with Ben, but did not hesitate to do with Miles. Peter was there to mentor him, watching the young man grow into a web-slinger that he could trust to defend New York and even exceed him in some ways. In contrast, every appearance Ben has made in Peter's life has come with some sort of upheaval. Miles' relationship with the original Spider-Man is far more amicable and as a result they've spent a great deal of time together, either because Peter was teaching Miles how to better use his abilities or they were off saving the world together.
Taking this into consideration, it makes perfect sense that Miles would know that it was someone else behind the mask. Miles knows Peter's voice so well because of all the time he spent with the original Spider-Man being mentored and developing a friendship. As a result, he was able to pick up on the slight tonal differences between Peter and Ben, which is remarkable considering that no one else has been able to do so.
Currently, only a handful of people are aware that Peter Parker is in a coma. His closest family members such as Mary Jane and Aunt May know, but beyond them only a few are aware that he's been hurt. It's not too surprising, considering that Spider-Man is largely a solo hero who doesn't usually ask for help. This is because he has always been one to shoulder his responsibilities by himself. However, this has come at the cost of isolating himself from other heroes who might be able to help during crucial moments, or in this case, provide much needed medical help.
Now, another hero has been made aware of the grave situation Peter is facing. Fortunately, Miles is a little more active in connecting with the superhero community than his mentor, already being part of a team of young heroes. He could very well be the messenger that spreads the word about Spider-Man's medical condition and sees to it that the webslinger gets all the help he needs.