I find the Android app Fing to be very useful. I don’t know if there is an iPhone version. It will do an nap scan of your lan and tell you what most of the devices are hardware and OS wise. It will tell you their hostname if available.
For the few that it can’t ID, a search on the MAC can at heart tell you the manufacturer of the Network device which can be informative.
When all of that fails, you just need to go through the process of elimination. Hopefully you are only looking at a couple unknown devices by that point.
But I completely agree with Using. You should know what devices are on your network. Once you do figure it what they all are, update Fing and periodically scan to look for rogue devices.
Indeed can’t recommend Fing highly enough.I use it on my home, work and parents networks. I have it installed on various Android devices and an ipad at work (the ios version is still good, but not as good).
I also joined the crowd funded Fingbox they developed (recently arrived ) which add some nice features.
/seeming shill over
Too bad has taken the route of now charging for premium subscription for devices they have already sold. I have a new OH project that I completed yesterday that provides notification services when new device joins the network. I recall Fing wants $59/yr for basic services going forward.