The antenna in this case is an M-Squared 40m4LLDD. It weighs 142lbs and has a 42' 2" boom. This is just a bit big and heavy to just pick up and find the balance point... plus the boom droops some when not supported as do the elements so it is difficult to just pick it up high enough by hand anyway.
Here is a way to ease assembly and balancing of a large antenna. The elements and boom are assembled separately in most cases. Once they are all together set up 2 tripods in the assembly area and put the boom on them. The tripods I use are made out of 10' pieces of old tv mast, but any reasonably decent pipe will work well. Chain link fence top rail is also a good material. Three pipes, a bit of rope, and an old Boy Scout Handbook (or Google for "Tripod Lashing") and you have a very serviceable tripod. With a bit of practice it is easy to raise and lower the boom and elements to comfortable working positions or even to walk the antenna around the area to get it ready to lift. After adding the elements to the boom and adding any other significant hardware you need to find the balance point. To do this I use a third tripod and a mast as shown below.
First, set up the third tripod over the boom and insert the mast through the boom to mast plate. It is possible to do it above the center tripod also, but this arrangement is inherently stable so I prefer it over that method. Tie the top of the mast to the top of the tripod and stick it down into the ground a bit so it is nice and solid and vertical. Note that there is only one U bolt on the boom, and it is loose so it is easy to slide the boom through it to balance. Also note that the boom brace ropes are slack.
This is the whole setup with the boom still supported by the two tripods near the ends.
Carefully lower the two tripods, making sure that the mast is supporting the weight of the boom. Be careful that the element tips don't dig in to the ground. Depending on how high the boom plate is off the ground you may not have to completely lower the tripods, you just need to get them down enough so that you can check the balance.
Mark the balance point at the one U bolt. Insert the second U bolt and THEN slide the boom over so the balance point is between the two bolts. At this point you can also adjust the boom to mast plate so it is perpendicular to the elements. Either use a carpenter's level or with this antenna I just used the element brace posts as a vertical reference and aligned them to the mast then tightened down the boom U bolts.
Now just raise the two assembly tripods back up to support the ends of the boom. While the temporary mast is in place the the boom is level you can also set the boom brace ropes to the right lengths.