A number of the medications can be liver toxic for some people, so it's always good to get liver function tests periodically. I've not heard of routine checking for fibrosis during treatment though, as problems usually show much sooner in the liver function tests - the liver is usually very good at signalling if it's under any stress by raised liver enzyme levels.
My liver didn't like Tamoxifen except at low doses, and my ALT shot up very high. This may be because I have Gilbert's syndrome (a genetic condition I only discovered much more recently). This means your liver is slower to process and remove some drugs from your system, and hence you can end up with higher effective doses of them in your system.
I mentioned the high ALT to a radiologist looking at a full body MRI scan result which had been taken at that same time (for unrelated reasons). He could use the scan software to calculate my liver fat, and it was 8% which is a bit high (should be ≤5%). If you have any MRI scans which include the liver, a radiologist can work out the percentage of liver fat at the time.
A couple of years later, I came across a mobile Fibroscan in the town centre and got my liver checked there. This measured my liver fat at 225dB/m (CAP, different method), which I think is <5%, so my liver had recovered.