Anytime you compile, the IDE reports how much is used.
Example from a '328P (Uno):
Sketch uses 5604 bytes (17%) of program storage space. Maximum is 32256 bytes.
Global variables use 575 bytes (28%) of dynamic memory, leaving 1473 bytes for local variables. Maximum is 2048 bytes.
The first line is flash memory, which the is the 32K of storage available less the bootload code (hence the maxiimum is less than 32 * 1024, or 32768).
The second line is static RAM, where the variables and the registers that are changed as the program runs are stored.