How do you make sure you have enough yarn for a scarf?

Get a scale and weigh two things:

The weight of the ball of yarn you have left.

The weight of the work that you've done so far.

To be the most accurate, slip your work onto a piece of waste yarn so you don't have the weight of your needles to factor in.

With these two numbers you can calculate how far along your WIP is and how much longer you ought to be able to knit.

I think you are asking whether you can knit the ends first, toward the middle, and then join the middles when you run out of yarn. The answer is, yes, you can. The joining would be done with a Kitchener stitch, the same way the toes of socks are joined. See KnittingHelp.com (http://www.knittinghelp.com/videos/knitting-tips and scroll down to "Finishing").

On the other hand, you may be the victim of an optical illusion. Often, I have looked at a half-finished scarf or afghan and thought, "Oh, gosh, I won't have enough yarn!" Yet, when I finished the whole item, it was enough yarn. It's happened to me so often that I know now to just relax, measure with an actual ruler the length I have, and compare it to the desired finished length. A scarf for a man can measure anywhere from about 50" on up, with 60" being the approximate right length for a dress scarf for a man in the 5'10" to 6'2" range. If I have 30 inches and about half or more of the yarn left, I'm good to go even if the 30" piece looks way too short to be half way.

As you gain experience, you will learn to check both the weight of a skein of yarn and the yardage. Two skeins of the same weight can have drastically different yardage, even if they are both listed as "worsted" or "Cat 4." Usually, buying an extra skein is no big deal. Most yarn shops will take back unused skeins if they are returned within a reasonable time (each shop will determine what's reasonable) with a receipt. Some will give a cash refund and some will give credit toward a future purchase, so ask up front. My local shop will give only credit, BUT they will also hold one or two skeins of my dye lot for 30 days to see if I need the extra. At that shop, it makes more sense to buy what one thinks one will need and then ask them to "reserve" a couple of skeins. A quick phone call will allow them to return unneeded skeins to the shelf or put them in a bag with your name on it for quick pickup.

Right now if you have access to the store you purchased the yarn at go and purchase more and keep the receipt so the full skeins you don't use can be returned. I am not sure what you can do or how far you have gotten, but continue until you run out of yarn. If it is long enough no problem. If is not get more yarn in a contrasting color and put that on each edge. Hope this helps. By the way check out the site

http://www.knittingfool.com/

under references and also if you have time see what to offer knitters.

a good way is to weigh a scarf you already have then weigh the yarn if it's close to same amount or more you have enough. you can also just knit a skinny scarf that will provide lengtht and an average scarf takes about 1 skien if crocheted and maybe a little less if knitted.

You have some excellent answers already. I just want to add that if it is really not long enough you ac get some good ideas for clever and creative additions on the ends from books like Knitting on the Edge, which you can probably get for free at your local library.