what tool to use for smd mark code to part number
For some devices there is the SMD Codebook - however there are so many new SMD devices coming out all the time it's impossible for it to be kept up to date. It's often good for things like SMD transistors, but rarely for chips.

As mentioned the service manual / schematic for the unit you are working with is the only sure-fire way of knowing exactly what it is.

Identifying the package can also narrow things down somewhat - for instance a capacitor will only ever be in a small number of packages and be instantly identifiable (unless it's a black tantalum capacitor that looks just like a diode, when it can be confusing). So a picture of the device posted on this site (you'll see there are a lot of "what is this component?" questions around) can get you moving in the right direction.