Even simpler is to take a tape measure or yardstick and measure the total length along the Hem.

Please remember though that very few kilt will have exactly 8 yards of fabric in them.

Depending on your hip size and the size of the Sett (or one full repeat of the Tartan pattern) your kilt could have as little as 6 yards and as much as 9 or 9.5 yards and still be referred to as "an 8 yard kilt".