Nope, a decker cannot get a direct yes/no to whether someone is a technomancer-- That's kind of the point, a TM is not a registered, normal part of the matrix that can be easily recognized. They are completely natural there, but are still in many ways an anomaly, and the matrix doesn't have set parameters for "can use resonance y/n" to be found on a TM's Living Persona with a simple Matrix Perception test. A decker has to use context, which isn't always that difficult, or else see the TM do something that doesn't make sense with how the matrix is coded (like "compile" a sprite or use a CF that totally bypasses how the matrix has to do things) and assume it's the resonance.
One good way to tell is if the persona has no cyberprograms running. A decker will always have some programs running, potentially more than their Device Rating. There's no reason not to. If you know a hacker never has any programs running, but who should know what they're doing, then that's a clue. As well, I believe you could normally get what model the device you're looking at is-- Most devices have that kind of information available for advertising reasons, and decks would for tech support reasons too. A TM will obviously give different information on that kind of thing, or else make it look like it was intentionally obscured. Not enough on its own, but again it's about building context.
As for being recognized by another TM, there's resonance signatures as others have mentioned. Keep in mind though, Matrix Perception is not reactionary like other perception tests can be. The second TM wouldn't get an automatic test to see if they notice a resonance signature, they'd have to analyze the device the CF was used on or the host it was used in. However the 2nd TM would probably just use Matrix Perception on the first TM at some point, and could find out that way (as TMs always have their own signature on themselves at all times).
