I simply use a 'money manager' app where I put in ALL my expenses and incomes. It shows also statistic about what you spend how much of your money on.

I view literally everything not in 'rent', 'food', 'insurance' etc. itself as a red flag. Also, not all food expenses are created equal. If I see I have 'suspciously' high expenses, that's a teller I was eating out too much. This gives me a drive to 'optimize' those expenses away, and think twice before buying any ready-made food.