what fact table is being used to creat dataset

If you open your dataset, you can use the "Design Mode" button to switch from the graphical designer to the text editor.  This allows you to have a look at the MDX query behind the dataset and you'll see all dimensions and measures being retrieved.

Here's an article that should give you a bit more insight in this: https://www.experts-exchange.com/Microsoft/Development/MS-SQL-Server/MS-SQL_Reporting/A_2320-Your-First-OLAP-Report.html

I want to know what fact (measure)
table is being used since I have more then one. The MDX only says [Measures].[FieldName]

Ah, well in that case you'll need to look at the cube's design (SSAS project).  If you don't have that project, you can open an existing cube by launching the BIDS and then File > Open > Analysis Services Database.

The following assumes you're looking at the OLAP DB in the BIDS.

If you open the cube and select your measure, you can then use the Properties pane to look at its Source settings.  The table name that you'll find there is the "friendly name" as specified in the Data Source View.  To get the physical name of the fact table, open the Data Source View and locate your table.  In its properties you'll find the actual table name in the Name property.

So I guess that means there isn't really an "easy" way, but at least there is a way :)

Any way that i can tell when i am in bids in a SSRS project?

OK Thank you.

If it's really so difficult to remember where the facts are coming from, perhaps it's an option to rename them to make it more clear?

Or to use measure groups?

I know, both require changes to the cube, so it may not be possible in your situation.  Just wanted to share the options.