By your water readings. And you may need 2 test kits to get all your readings. Nitrate , nitrite , Ph , ammonia , phosphate and calcium are the most important. And test in the morning around the same time for most accurate readings as daily tank activity can change readings.
A good amount is 10 to 20 % each month. If you do 20 and your reading stay stable then your fine.
people say the rule of thumb is 20% every other week. or 10% every week like stated above. these guys are correct but it all depends on the set up.
i have a 90 gallon tank with a 25 gallon sump. i only do 25 gallons every month to month in a half. but keep in mind i have a fuge in my sump with chato and a deep sand bed in the sump as well LR rubble, pound of LR per gallon, ect. it all depends on how you have your filtration system set up as well as your bio load (number of fish). all tanks are different so you need to test your water a lot to get a good idea how your system is working.
i have a buddy that has a deep sand bed in display tank, deep sand bed in fuge with mud, chato, 100 lbs of LR, a good skimmer with 4 fish in his 90gallon. he only needs to do a water change every other month and some times once every 3 months. i know sounds tripy but its true. his tank is sps dominate with some soft corals and his tank is growing coral like crazy and fish are doing quite well.
like i said every tank is different so you need to test your water frequently to get a feel for your tanks personality. good luck