It is very important to measure the lift from the marketing campaign. This is essential to understand how much more products or consumers are converting due to the marketing/advertising efforts.
A basic test/control methodology should be used to find out the lift in conversion rate due to advertising. This is easy to understand with an example of the grocery store - how likely is someone to buy a product if he/she has not seen any advertising and then how likely is someone to buy a product if he/she has seen advertising. The % difference between the two is the lift from the advertising.
In simple words you found out the purchase intent without the advertising (Control group) and then with the advertising (Test Group). The % difference between the two provides the lift.
How easy is it to implement or execute this?
For Online marketing, it is not hard at all. Comscore has a panel of over 1MM consumers whom they monitor their online behavior. Their solution creates a test/control group based on Ad exposure and conversion.
DoubleClick also has a cookie based solution to segment consumers who saw the ad and those who did not and then calculating the lift due to advertising.
For offline marketing, there are mathematical models which have been used in the past to calculate the lift.