How will I know if my Airport extreme A1408 is not functioning well?

The 2.4 GHz band is by far the most crowded and congested place in WiFi. If you live in an apartment or another type of close quarters, it is possible that there are just too many other networks around you that are using up the available channels for WiFi. It's very likely that other users are having the same kind of problems that you are.

If you have not already done so a few times, try powering off the AirPort Extreme for a few minutes, then power it back up. This will force the AirPort Extreme to scan available WiFi channels and try to pick the best one to use at the time.

If things are still acting up, there is not really much more than can be done. Sometimes, resetting the AirPort Extreme back to factory defaults an then setting it up again will help, but there are no guarantees that the hassle of doing this will result in a solution.

The average useful life of a WiFi router designed for home use is about 5 years, so there is that to consider as well. As LaPastenague has mentioned, your AirPort Extreme is 7+ years old and as much as 9+ depending on when it was placed into service. If things have worked well up until this time, you have definitely had very good luck with your AirPort, but it is really time to replace the product.

My download and upload speeds have been slow on my 2.4 Ghz.

Does 5ghz still work OK?

The Gen5 Extreme is now 7-9 years old.. Honestly the question is not how to check it but how quickly can you replace it.

Anyhow what exact speed are you getting on what exact client? What is the link speed at each end?

Airports show link speed by hovering your mouse pointer over the client name or IP/MAC address.

There is quite a bit of info there including band, signal level and link speed. Actual speed will be about half link speed in ideal world.

What you find as items age the wifi signal levels drop down and it does not get the range.. often up close it will still work ok.

Clients have too many options to show exact details. But Mac is easy.

Hold down the option key while you click the wifi fan.

Note the best possible wifi speed on later Mac with Gen5 Airport on 2.4ghz is 217mbps. On earlier gear it will be 150Mbps .. or a bit less.. 140 say.