How "SWAG" Current-/Power consumption of electronic circuits?

Hi folks,

yeah! The question is simple, the answer may not be:

"How do I "SWAG" Current-/Power consumption of electronic circuits which I have not built/developed?"

...being too lazy to exactly analyse, (re)calculate all circuitry details or measure the power rails... How do I go about making a rough, but educated guess, in order to ensure a certain power supply will be sufficient for the circuit?

Thanks a lot for advice!
Regards,
Winfried