Even if you haven't take a starting gravity you can still use a gravity reading or two during the process to tell you what's going on. Your instructions should give you a starting gravity, especially if you're doing extract which is fool proof, that WILL be your gravity as long as your volume in the refermenter is what the recipe says it should be. So if you take a reading, and the number is lower that that, you know you have fermentation then.
Or you can take two over a couple days and if it's changed you know you have fermentation.
Because that is the biggest question we get on here. Is my beer fermenting, (usually followed by "because my airlock isn't or has stopped bubbling) so a number lower than the SG will tell you.
Your recipe should also tell you what the FG should be. So taking a reading if you want to know where the beer is at, will tell you how close you're getting to terminal gravity ("Hey, my airlock stopped, does that mean fermentation is done?)
If the fg for your beer is supposed to be around 1.013 and it's only been 3 days or so, and your airlock stopped, but your gravity is reading like 1.030, then more than likely fermentation is still going, but less EXCESS co2 is being produced, but fermentation is still happening.
Taking another one a day or two later will tell you if it's stuck or finished.
It's a dianostic tool. It will tell you where your beer is at and if it's stuck or finished, just by how you interpret the numbers.