Does anyone know how to recognize true "feed cloth" ?

You mean feed sacks, correct??
If that's what you mean they are very durable fabrics. But the only way I can tell for sure is to get them from an antique place, flea market etc.
The reproductions are thinner fabric, in my experience. That's the big difference for me.
Hope this helps!! :D

Most of what people call feed sack fabric is actually flour or sugar sack fabric. They're a finer weave than feed sacks. Feed sacks had feed for animals in them so they weren't necessarily all that fine of a weave. Flour and sugar sack fabric got used for clothes and the scraps for quilts. Feed sack fabric got used for quilt backings and things the looser weave was okay for.

The only way I know is I have my grandmothers flour sack material. I have had it for 40+ years and she died in the 60s at age 80. I inherited the material. I have mixed feelings about putting it in a quilt. I love to get it out and look at it.

I will follow this thread

I have a quilt I am "rescueing" for a lady and I need to find some feed sack fabric for repairing a few holes. I will watch this post too. There were different fabrics for each flower so I can replace the whole block with a new flower when I find feed sack fabric. Any ideas?

I was reading other posts and found one that Craftybear put several references to where to buy feed sack fabric. So clever