There is a simple way, when you have the LXF-Files from Lego Designer (LDD) or another 3D-Lego-programm-file. But you need an Rebrickable-Account.
First you have to create a new part list, for the parts you already have
Go to www.rebrickable.com
Go to your Profil (small body icon on the top right) -> "My Parts"
Click "Add A Part List"
Give it a "Description" like 42055 Bucket Wheel Excavator
Choose in the drop down menue "Include these parts in Build calculations"
And Click "Save Details"
When you have a standard Set - like the 42055 Bucket Wheel Excavator
Go to "Sets" -> "Search Sets"
Search for the Set, e.g.: "42055"
Open the Set from the result list
Then you have to find "Add to My Parts (Part out Set)" - you will find it on the bottem right over the Comments-Section
Now Choose your created Part List from the drop down menue
Click "Part Out Sets"
When you have your own MOC or MOD with LXF or other 3D-File
Go to your Profil (small body icon on the top right) -> "My Private MOCs"
Go to "Add New Private MOC"
Give it a Description like Bucket Wheel Excavator MOD
Click "Save Details"
Now you can add the parts by clicking "Import from MPD/LDR/LXF file" - you can also use the other Imports
Choose your LXF-File and Click "Import" and follow the instructions
When you have created the Part List:
You can still add some missing parts by using "Find Parts to Add" (There you have to add it to your private MOC - which you have choose from the drop down menue)
Now you can go to "My Private MOCs" and Choose your created MOC from the list
Click on "Build this Private MOC" and you will get the results of the missing parts
Now you can export the missing parts for example to an XML-File, which you can upload to Bricklink to order the parts.
When you have done it once or twice it is a very easy an fast way to calculate the missing parts :)