If you know density and volume: (m = rho V)
If you know specific weight and volume: (m = frac{gamma V}{g})
If you know weight (force) and want mass: (m = frac{W}{g})
If you know force and acceleration (from Newton’s 2nd law): (m = frac{F}{a})
If you know momentum and velocity: (m = frac{p}{v})
If you know kinetic energy and speed: (m = frac{2K}{v^2})
If you know gravitational force and distance: (m = frac{F r^2}{G M})
If you know orbital period and central mass (circular orbit): (m) is not needed; use (T = 2pisqrt{frac{r^3}{GM}})
If you have a scale: read mass directly in grams (g) or kilograms (kg)