For all types of marketing, have a prospect management system that is set up to have a field where you enter where that prospect came from. If you can't track it by how they got in touch, then ask each one. Some won't remember or will give you multiple answers, but do the best you can to record where they came from. Later, you'll run reports by marketing method and know how many prospects you've gotten that way. If you also place commission amounts in the prospect (now client) record, those reports can tell you how much money that method has produced over time.
For internet marketing, it gets easier and far more accurate to determine how many prospects you've received from a web site, right down to which page or form they used. This is tracked with the web site statistics. A good site provider can also track your marketing by prospect name and the end result, purchased, sold, or removed.
