No really cheap way to see actual interference. You can use the wifi scanning tools but those only show you the router beacon message not the actual end devices. There is no way to know if the router is really being used or not. It also will not see anything that is not wifi. Things like baby monitor or cordless phones use the same radio channels but are not detectable.
What you actually need is called spectrum analyzer. Very expensive devices only really used by professionals.
Uniquiti makes wifi bridge equipment. For some reason they have the ability to actually run as a spectrum analyzer just for the wifi bands they cover called airview. You can see things like a microwave oven that runs on the same 2.4g frequency. Problem is you would need 2 different units one for 2.4g and one for 5g. The cheapest ones that have the feature are nanostation loco m2 & m5 units. They cost about $50 each.