It is very common that you go for car servicing at dealer workshop with minor running repairs but you get a bill with some parts changed without your knowledge. Most of the car owner has similar complaints.

Dealer workshops have many targets to meet. Targets of spare parts orders in a month. So, service managers get incentives to meet that target. It is obvious they would try to sell same parts to every car owner even if that is not required. Incentives to service manager leads to mal practices at workshops.
To avoid these problem, here are few things you can keep in mind.

1. Car Check up
Always go for car check up first and get assessment with estimated cost. It is in the report that which parts runs how much time. For example, service manager would say your break pad needs to be changed. There are chances that present condition of break pads may not worse but they recommend it do it now. Ask them few questions. You are the better judge!

2. Estimated Cost
All good car workshops offer estimated cost during car check up. Ask for labour charges, that makes difference as it varies from workshop to workshop. For your knowledge, dealer and independent workshop labour cost has 25 % to 50% labour cost difference.

3. Test Drive
Every good car workshop offer test drive after servicing. You must ask for test drive. If you find any problem during test drive, you can point out.

4. Extra Parts
Always go for genuine spare parts. Maruti offers genuine spare parts are Maruti Authorized Service Station and have strict guidelines to their service station of not using any spurious spares. Other brand Authorized service stations also have genuine spare parts, but ask to car workshop to show you spare parts they are using.

Next time your car mechanic suggests you replacement of new parts, you must have assessment ready if that is required or not. There is no formula to know but when you get assessment from 2 car workshops, then you may have fair idea.