How can you determine which of any two halogens is more active?

Halogens are F2, Cl2, Br2, I2.

Add F2 to chloride, bromide, and iodide salts.
Add Cl2 to fluoride, bromide, iodide salts.
Add Br2 to fluoride, chloride, iodide salts.
Add I2 to fluoride, chloride, bromide salts.
The more active halogen will displace less active halide below it.