I do not believe that there is any way to find out from the router what default gateway is configured on a PC (whether the PC is connected to the router or not). To determine what default gateway is configured you would need access to the PC either by being physically on the PC or through some remote access software such as Remote Desktop. Or perhaps there is some management software that can monitor PC configuration of default gateway. But not from a router.
I'm not sure what you mean by "that is turned off from a router within a LAN". Do you mean it is trying to use some other router?
If so, you could try monitoring its traffic. If it can find its router, then it will send anything destined for off-net to what it thinks is the router. Then look for the destination MAC address. OTOH, if it can't find its router, then it will be ARPing for it every time it wants to send something off net.
On the other hand, it could be ARPing for everything, on or off net. In that case it has no gateway configured at all.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...