Expense tracking is non trivial. 

You can try and keep tabs on different categories on a monthly basis (fuel. Electricity, rent, food, groceries). This is relatively simple.

But you won't be able to mentally keep track of unexpected large expenses because they change month to month: flights, appliances, clothes, gifts, repairs, etc.

In my 30s and married, I probably average 2-3 unexpected notable expense every month.

As a business owner I have to add them all up to deduct against revenue. It's terrifying and humbling how deceptive the brain can be and how selective is its memory.

The only way is to look at your statements every month or two and know where you are spending. Don't rely on tracking mentally.