It all depends on what your marketing effort was supposed to have achieved Pamela!

Did it happen or not?

To deploy any meaningful measure you must determine 4 key objectives, what final result are are targeting, how much, by when and on what terms.

None of these criteria are defined in your question.

Great question!
Firstly you need to decide your aim for marketing. Is it about branding or conversion?
If it's about Branding, There is no actual way of tracking it if it's now digital. If it's digital then you can track it by impressions.
If your aim is conversion. You can make fill out a form (asking about where they know about you) at the time of conversion so that you can track.
If your marketing is digital then it is very easy, you can use google analytics and try to track customers at the time of conversion.

I usually compare some key matrix with last month. Like new visitors, returning visitors, organic traffic, referral traffic, goal completion or leads.

Marketing is designed to change the behavior of your audience. Taking them from inaction to action. Successful (and I would argue all) marketing should take prospects from not buying to buying. This can be taken in one giant leap (not often) or a sequence of events.

The ultimate measuring stick is sales - ie dollars in your bank account.

Is a campaign really successful if it doesn't reach your end goal?

You could for example have prospects go from not clicking on your facebook 'likes' to clicking. These likes are great, however meaningless as a marketing tactic if your next step is not in place to ultimately take them to a place where they are taking action that ends in a sale.

The most important thing is setting your goals ahead of your marketing efforts (or at least setting them before your next marketing effort). You need to have things to track against, however, you also need to select the right things to be tracking.

What would items would really indicate whether your business was growing, flat or struggling? Those are probably the best key metrics that you should be tracking. Every business (and every industry) have different success indicators.

Goals are important for this one, I check the goals I set.
I also use data from before I started the new marketing activity, or same time last year (whichever is more relevant).