Probrewer here. BioBeing is correct, and he makes the important distinction about extract vs all grain. if extract then you can easily calculate an accurate OG later by simply knowing the recipe. the only assumption this makes is that you're not SO newbie that you didn't warm up the can of syrup to get all of it out, didn't throw out significant portion of syrup with can, etc. this is common with somebody's first-batch-ever, but usually by their 2nd batch they figure that out.
if all grain (which i'm guessing since a quick google search would tell you same about extract): sorry, you're kinda out of luck. now previous posters were correct to point you back to your recipe, and the yeast, and that strains typical attenuation level etc, in order to calculate a theoretical alc%. however, since you also mentioned that you're new to homebrewing, if i was a betting man i'd bet against an estimate generated this way. This is because the most common negative result that newbie hbers get is a lower than expected drop in grav during fermentation. this is rarely a function of the yeast chosen, more commonly caused by poor "conversion" in the mash. conversion means the level to which you successfully changed the unfermentable sugars originally found in the grain into ferm sugars. this is usually a function of temperature. ALWAYS take great care to stir the water and then take a temp reading right before you add to your mash tun. and if you're really new (ie, new to your own equipment) then you should then stir again and read temp again after water is in the tun to develop a good sense of how many degrees the water drops by going from the hot metal container you heated it in to the cool container in which you mash. (a room temp cut-keg will drop 6.25 gallons of hot water reliably by about 8 degrees. a plastic cooler will drop it less, but still a significant amount.)
the second common cause of poor fermentability is bad liquor-to-grist ratio. Newbies often wonder why they need so much water in the mash to begin with and so cut down the volume of water. This does in fact delay enzyme denaturing, but at the cost of reducing their mobility within the mash matrix. Result, poor conversion. Conversely, other newbies will think they can eliminate the need to sparge if they just add the sparge water at the beginning of the mash. This also results in poor fermentability, in this case because of pH. The excess water drives the pH of the mash way out of the operating range for the enzymes. Bottom line with liquor-to-grist, dont stray too far from the rule-of-thumb ratios you see most often quoted, typically 1.25 1.5. I tend to shoot for about 1.4 quarts-per-lb myself.
after poor fermentability/conversion, the next most common culprit in lower than expected grav drops is poor oxygenation. The amount of dissolved O2 in the wort at pitch-time directly correlates to the amount to which the yeast cells multiply. If you have poor oxygenation, the yeast will only multiply up to a limited amount, and then when they switch over to anaerobic phase, there wont be enough of them to do the job. Or they will, but it will take months for them to exhaust their food supply. When hbers claim a stuck ferment, this is commonly the cause. Its not actually stuck, its just going inconveniently slow.
Another dimension to vex a new brewers ability to back-calculate alc% is the OG itself. If its your tenth batch of the same recipe and youre preboil grav was always pretty close in the first nine batches, then you can be pretty confident that #10 will be the same. But if its your second or third batch, then youre most likely still getting the mechanics of sparging down, and your pre-boil gravs are likely varying significantly. Speed of runoff, heat of the sparge water, whether or not you use some method to divert the downward momentum of the sparge water so as to reduce tunneling which in turn causes preferential flow within the settled grain bed
all these factors are potential sources of poor runoff efficiency, and it usually takes a brew or two EACH to grasp why these matter. And btw, dont beat yourself up for not somehow forseeing these, THEY ARE NOT INTUITIVE! Some are COUNTER-intuitive, in fact. And by the same token, dont think its all too complicated for you to learn, it just takes practice. My mom is a church secretary and doesnt have a scientific bone in her body, yet my brother, the electrical engineer and all-round smarty pants, cant figure out how to reproduce her basic gravy recipe. Its just trial and error.
But back to poor runoff efficiency, if you collected less sugar from the grain per volume of wort than you assumed you did, the result could be a very nice fermentation leading to a drinkable beer
that has a lot less alcohol than you think it does.
Hope all this helps. And remember, this hobby can be as complicated or as simple as you like it. Ive met very accomplished hbers who came at it from very different angles.
