Yes, it is one or the other.

Our head units ("stereos") drive the amplifier at speaker level. That means you can cut wires to the amp and put a speaker in place - if you get sound, the amp is bad.
If you don't get sound, the head is bad.

You can also use an RMS voltmeter set to the 2 volt AC scale, and measure the output at the back of the head - it's not as easy to interpret the meter readings as it is just listening to a speaker, but it can be done.

If NO speakers at all produce sound, I'd think the amplifier is bad. Yank the carpet / trim out from the passenger footwell, if you see water damage or corrosion on the amp connectors, you know what your issue is. The FIX is a new amp, put in a new head and wire it for four channel sound while you are at it - and you will be HAPPY this happened when you hear what the system CAN sound like!