How to test if you are sewing an exact 1/4" seam allowance
1. Take 3 pieces of fabric ... in this case 5" square. Lay them out in a row.
2. Place the right hand square over the middle fabric. Align the edges.
3. Sew a 1/4" seam.
4. Set the seam with a hot iron then press the seam towards the right hand fabric - the blue square in this case.
5. Place the left hand fabric over the middle fabric. Aligning the edges.
6. Sew a 1/4" seam.
7. Set the seam. Press towards the left hand fabric. The blue square in this case.
8. Using a ruler measure the size of the middle square. It should measure 0.5" smaller than the original size. In this case 4.5"
9. Check the size of the block at the top, the middle and the bottom. They should all measure the same. (4.5" in this case)
10. If the middle block is an exact 0.5" smaller than the starting block width you have sewn an accurate 1/4" seam on both sides.
11. If the block measures less than 4.5" (in this case) your seam allowance is too big and you need to make your seam allowance smaller.
12. If the block measures more than 4.5" your seam allowance is too small and you need to make your seam allowance bigger.
13. Repeat this test until the middle block is exactly 0.5" smaller than the starting block size.
14. To help you achieve an exact 1/4" seam allowance consider buying a specialist foot for your machine. There are several types available. This one has an edge guide which is useful for butting your fabric against.
Be aware that even with an edge guide your seam allowance will not be accurate if you do not keep your fabric aligned with the edge of the guide. Checking the seam allowance at the top/middle and end of the block also ensures you are sewing a consistent seam width along the whole block or strip set. It is common to see slight variations in seam allowance at the end of blocks or strips if we are tired or not concentrating. See this video here.