I have excel file with 15 sheets

Is there a way that I can check if any of the fields are linked to another field or sheet (without checking each field)?

Thank you

Go to Tools->Formula Audinting->Formula Auditing Mode (or use Ctl+`)

This will show all the cell formulas in the cells, rather than the values.
It's easier to scan that way, but you're still stuck with looking at every cell.

This works for fields within a spreadsheet (not sure how it displays cross-worksheet dependencies):

Tools/Formula Auditing/Trace Dependents

This works for fields within a spreadsheet (not sure how it displays cross-worksheet dependencies):

Tools/Formula Auditing/Trace Dependents