Plug your computer to router with LAN, if LAN speedtest same speed as wifi then no need for wifi upgrade.
This solution is only valid if all you need your wifi connection for is to route traffic from the internet to your computer. If that’s the case, then this will show if you have any speed degradation once the packets hit your network. If that’s all you’re interested in then you may not even need to do this. Compare the speed test results at a site such as broadbandreports.com with what your ISP is rating your internet connection’s bandwidth; if they’re comparable then you’re fine.
However, if you need to do things like stream video files from a media server on your network, then you may find your 802.11g connection isn’t up to your requirements. If you directly connect one pc to your router and wirelessly connect another pc, and then transfer a file from one to the other, you can see what the transfer speed is. You can then compare that to what you would need for whatever LAN purpose is at question. Obviously that will vary by requirement; for example, streaming SD video will not be as demanding as streaming HD, etc.