The liver is the site of metabolism of many drugs. In return, it gets affected by many medications.

The most important ones being

Statins for lipid lowering,
Amiodarone for control of VT and SVT
Methotrexate - for psoriasis, RA and chorio-carcinoma
Rifampin and INH - used for tuberculosis

One of the facts commonly tested is that INH hepato-toxicity is not checked unless the patient has symptoms related to the liver. This is so because even if patient has elevated trans-aminases, one does not stop the drug.

There are many others but these are the ones most commonly tested.