Hi Friedlieb,
I guess it comes with time and experience working with those respective modules, however you can apply a few techniques to help you get your answer.
Some of things I do usually in this order are:
1) In SE11 use the graphic display (CTRL + SHFT + F11), which use the foreign key relationships specifically to that table.
2) Perform a where used list table on "Views". I usually look for SAP standard views and then check how they have defined the join conditions. Sometimes I even find a view that does exactly what I want so I just use that.
3) Check the SAP Transaction and use F1 to check the field name. Quite often the table has been used as the structured type for the fields displayed.
4) Read Review SAP Code in BAPIs and see which tables are being updated
5) And obviously ask a colleague with spefici functional knowledge or check on SCN
P.s. A good table for Sales Doc Flow is VBFA.
I'd be interested to hear what others do in this space also. As far as I am aware there is not straight forward solution.
Regards,
Katan