You'll still need scales, but can manage with smaller or less-suitable scales using the method below. I wrote this a couple of years ago for our club:

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The weight and balance of gliders is critical to safety and handling in flight. The existing weight and balance sheets of many gliders are out of date or erroneous. Repairs, alterations and painting change the situation but are not always accounted for. Pilots who are smaller or larger than average adjust the seating, changing the CG location to an extent that is unknown unless the glider is weighed with the pilot seated.

We now have access to two scales of the sliding-weight type that are usable for the measurements needed to ascertain the weight and balance. One is a platform scale that measures to over 1200 pounds, the other to 130 pounds with more precision. But there are practical difficulties in a direct measurement of the assembled glider. The glider needs to be leveled (in pitch).. The platform is elevated, making it harder for the pilot to get into the seat. The tail would need to be elevated higher yet to level the glider. The vertical part of the platform scale may not fit under the wing. Any fall from the elevated perches may damage the glider. Electronic platform scales with no vertical parts are available for cheap these days, but their accuracy and longevity are in doubt.

Recently we came up with a trick way to accomplish a full measurement safely with the mechanical scales on hand, and successfully used it on one glider:

First weigh all the glider parts, separately. One wing at a time on the big platform, with padding. The fuselage needs a cradle that can roll out of the trailer, some gliders have that already, for others would need to improvise such. Set up a simple ramp and roll the the fuselage onto the platform scale. Put the small scale under the tailwheel. Weigh both points at the same time and add up the two numbers. The fuselage does not need to be leveled at this time, we're only seeking the total fuselage weight. Weigh the canopy separately or with the fuselage. Remember to include all the small pieces: removable tail, spar pins, battery, etc. Weigh the dollies and padding on their own, and subtract, to get the net weight of the various glider parts. Sum up the parts and get the glider's total empty weight. Compare with the pre-existing weight record.

Second, fully assemble the glider. With the main wheel on the ground, and wings held level, elevate the tail on top of the small scale to level the fuselage as per the manufacturer's directions. (It is safest to elevate the scale itself on top of a wide base.) Record the net weight at the tailwheel. Here's the trick: the weight at the main wheel is not measured directly, but can be computed by subtracting the tailwheel weight from the total empty weight (known from the first step above). From these numbers the glider's empty CG can be computed. Compare with the pre-existing record.

Next, weigh the pilot (with typical flying clothes, and with the parachute if it is to be used). Repeat the tailwheel weight measurement of the assembled and leveled glider, with the pilot seated and canopy closed. Again, the weight at the mainwheel can be computed by subtraction (empty weight plus pilot weight minus weight at tailwheel). Now the gross weight and the CG location can be computed. Check that the CG is within the allowed range. Can also deduce the actual location of the CG of the pilot, compare that with the location assumed in the manufacturer's instructions.

The calculations are best done in spreadsheet software. If you need help, ask a mechanic or one of the club CFIs. Trick question: does it matter where the "datum" is?