Hi Keiji,
Generally I would set up a program that captures your source in the Person Source / Lead Source field (it is named a bit inconsistently in the UI but is the same thing). Information you can use to identify the correct source is:
- Any utm source data you capture from form fills
- Person being created from CRM
- Original referrer
- Original Search Engine
- Original Source Type
- Source Type on Person Creation activity
You define your own hierarchy of sources and stamp the field once and don't overwrite, so you keep the original source as a fixed value always. As you can see Marketo captures a lot of information, but you need to define for yourself which of these datapoints is the most important one to define what your source is.