Excel calculation order within formula
Excel calculates in a specific way within each formula. From the Microsoft documentation it will process from left to right but will process brackets (Aka parentheses) first. Excel evaluates in order of BODMAS
Excel calculation order between formula
Excel will determine which formula are independent of others and calculate those first. This includes formulae referring to cells that only contain values or forumulae that reference constants or volatile functions.
Finding the processing sequence
For starters, this is also known as tracing dependencies (and precedents). In Excel, in the formula tab on the ribbon you can access the audit functions that allow you to trace dependents or precedents. These will point you at the formulae that will be calculated before or after a given cell. This gives you the ability to step through the order so that you can ensure it's correct. For more detail on this I recommend this Microsoft article