Do you mean something like 100 m = ? ft or something like finding the unit of a quantity by using an equation? (Such as F = Gm_1*m_2/r^2, and find the unit for G)? Or is it something else?
I think I know what you are asking but I might be wrong. I think the best way to explain is with an example.
We want a formula to describe how long it takes a ripple moving through a liquid to die down.
We assume that the time taken is a function of the fluid's density , viscosity and the speed of sound in the fluid
Assume that the formula is in the form
Examining the units of each quantity
The formula must be such that the units are balanced
The indexes on the units must match up
If you solve this set of equations you find that b=1, c=-4, a=-1.
So we know the equation must be