You say its an ecommerce website, so I assume you are interested in revenue as a KPI.
You don't actually need a Goal setup to measure goals if they are based on page URLs, so if for example you have a /thank-you page after a user has transacted, you could use the Top Content report to filter down to those pages, and the number of PageViews will be the equivalent to transactions.
If you know the total website revenue per month, you could then work out an average value per transaction, to get a rough idea on value per month.
Once you have that, you could move to channel analysis - segment the pageviews of /thank-you pageviews per medium. Using your average value from above you can then attribute an approximate revenue each channel created for you.
Another tactic could be to segment to sessions that included your /thank-you page (or another page you think represents a valuable visit). This will segment to visitors who were customers, which if compared to other e-commerce websites will be around 3% of your visitors. You can then examine which pages have the most pageviews in this segment, which will be a list of your most valuable pages - those pages that eventually resulted in a visit to your "money" page. Your homepage will feature most likely, but the interesting insights will be down the list.
But its all a lot easier if you have ecommerce / goal tracking set up, as then you can use the segments already setup as default. I would generally recommend to implement this ASAP, through Google Tag Manager if possible to make it easy to tweak later.