Only attempt to open the seams of fully fashioned garments with linked seams, or hand knitted garments with hand sewn seams. Avoid trying to open the overlocked seams of cut and sew knitwear, as these cover cut edges. For guidance on identifying and opening the seams of machine-made knitwear, see Dawn Prickett's Recycling Sweaters for Yarn blog post (note: 'serged seam' = overlocked seam, 'crochet seam' = linked seam).
If you want to pick up open stitches from a cut and sew garment, you would have to just cut the fabric at the appropriate point, rather than opening the seams first (see open a row for more details).
For all seam types:
For hand knitted garments with hand sewn seams:
For fully fashioned garments with linked seams: