How can I diagnose if my it's my phone that has a problem with wifi? I have a Galaxy Note 4 - and I've been having trouble recently with the Wifi frequently disconnecting. 

What I would like to do is diagnose whether this issue is specific to my phone, before I send it back for servicing. 

Do you have any suggestions how I could conclusively test this?
I recommend connecting the phone to a different wifi network. The easiest way is to take it to the home of someone you know who has a well-functioning wifi, and log in. If a connection's successful, stay with it for a while. Use your phone in the same way as you have been, and if everything works as it should, your wifi is likely the source of the problem.

Another option is to test your network using a different phone or other device. You should log your phone out first, to get it out of the equation. If no problems occur with that device, your phone probably needs service. 

There are also public places, like restaurants, that have free wifi. You can test your phone in that type of setting. It would be my last choice, though, because large public networks tend to have problems of their own.