Is there anyway to tell 100% cotton from the cotton blends? I have been told by my local fabric store that all their fabrics are 100% cotton, but I am not sure...

Burn it.

Here are some ideas about the burn test, wrinkle test, I am not sure what else.
http://www.quiltingboard.com/t-98012-1.htm

You can do the burn test...find an ashtray or old bowl, cut a small piece of fabric, set light to it and let it burn. Look at the residue, if it has any stickiness it is a poly blend. Pure cotton burns fast and clean...no messy gunk.

Don't however do this in the shop!

Often the bolt ends have the type of fabric written on them, sometimes the selvedges. Generally though I would think the store-owner would be honest, otherwise they wouldn't get repeat business.

Thanks for your help, I will be burning fabric. LOL

if you burn the edge of a piece of fabric with polyester in it the edge will become very hard...brittle, crisp...(the plastic melting) cotton edges stay (scorched) but soft.

lots of times a fabric on the bolt may (feel) different...it is usually from the added chemicals-sizing...if you pre=wash your fabrics the cottons will be back to being nice and soft...feel better than a blend...a blend the hand does not change with washing.
usually on the salvage it will say who the manufacturer is and the fiber content...98% of my fabrics say 100% cotton on the salvage....i have seen fabrics wrapped on bolt boards they do not belong on...totally different fabric stated on end of bolt from what is actually there...i always check the end and open up the fabric and look at it, making sure i am getting what i pay for.

It is nice to get so much help. Y'all are great.

I burn test all the time and one word of advice...use tweezers or something to hold the fabric. Ouch!