A service bill is the one document most owners sign without reading. The line items sound technical, the amounts look considered, and the person handing it over is the same person who just spent three hours with your car. Reading it is a skill, and it takes about ten minutes to learn.
Start with the intervals
Most of what inflates a bill is work done early rather than work done badly. Engine oil on a petrol hatchback runs ten thousand kilometres in normal use. Brake fluid is a two-year item. An air filter is usually a clean, not a replacement, until it is visibly grey. If a line says replaced and your odometer says the last one was four thousand kilometres ago, that is the line to ask about.
The three lines to check
Labour is charged in hours, not in tasks — two jobs on the same subassembly should not be billed as two full hours. Consumables should be a small percentage, not a round number that happens to end in a zero. And any line marked as per customer request should be something you actually remember requesting.
Odo checks the bill for you
Scan a service bill and Odo compares each line against what the same job costs at other workshops nearby, then tells you which lines sit above the usual range.
None of this makes you an adversary at the service desk. It makes you a customer who reads, which is the only kind of customer a fair workshop wants anyway.