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If you are unable to connect to a network and suspect that the router's MAC filter is active, there are a few steps you can take to troubleshoot the issue from your client.

Check the MAC address of your network card: Make sure that the MAC address of your network card is not being blocked by the router. You can check the MAC address of your network card by running the command ip link show or ifconfig on Linux, or getmac on Windows.

Check the router's configuration: If you have access to the router's web interface, you can check the router's configuration to see if MAC filtering is enabled, and if so, check if your MAC address is listed as an allowed device. Bear in mind that Wi-Fi and LAN have different Mac addresses!

Check the router's status lights: Some routers have status lights that indicate if the router is blocking a device's MAC address. Check the router's manual to see if there is a status light that indicates MAC filtering is active.

Try connecting from a different device: If you have access to another device, try connecting to the network with it. If the other device is able to connect, the issue may be specific to your device's MAC address.

Check the router's logs: Some routers have logs that show which devices have been blocked by the router. If the router has this feature, check the logs to see if your device's MAC address has been blocked.

Check for other network issues: Make sure that there are no other network issues that might be preventing you from connecting, such as a weak signal, incorrect network settings, or a malfunctioning router.

It's worth noting that the troubleshooting steps may vary depending on the router's manufacturer and model, also, you may need to have access to the router's web interface or have login credentials to check some of the options like the router's logs.

There may be a way to determine if a router's MAC filter is active from a Linux Ubuntu client using the terminal, but it would likely depend on the specific router and its configuration. Here are a few possible options:

Use the arp command: If the router is configured to block the MAC address, it will not respond to the ARP request for the IP address. You can use the command arp -a to check the ARP table for the IP address of the router, and see if there is a corresponding MAC address.

Use the nmap command: You can use the nmap command to scan the network for open ports and services. If the router is configured to block the MAC address, it may not respond to the nmap scan, and the router's IP address will not be listed in the scan results.

Use the ping command: You can use the ping command to send ICMP echo request packets to the router's IP address. If the router is configured to block the MAC address, it may not respond to the ping request.

Use the tcpdump command: You can use the tcpdump command to capture network traffic. If the router is configured to block the MAC address, you may not see any traffic from the router in the capture.

It's important to note that the availability of these options, and how to check for them, may vary depending on the type of router, its firmware version and configuration, also some commands may not give you a clear answer, but if you see that the router is not responding to the requests, it might be an indication that the MAC filtering is active.