How can one tell the difference in the quality of fabric used for quilts. I have notice some fabric being thin while other are very thick. Also, what does it mean by "thread out of a fabric"?

i would think its the thread count in the fabric, if it feels thin my thought is its a weaker fabric. Alot depends on where you purchase your fabric.

I saw the same print in 3 plases. One was real thin, and it was the same print.

i've seen this happen as well.

different markets have different (quality control measures)
when a line is printed/ran it may be going to 10 different markets- all with different (starting fabric) and price base
so you can find the same (print) in the lqs, joannes and walmart- they may all have the same design but be very different fabric.
after a while you can tell by feel what you like- what you do not like-
it's not all the same fabric- its only the same print on the different fabrics.
when trying to decide if its (good quality) feel comes first- then actually look at the print- often in the (lesser markets) the print may be (off) crooked, the fabric itself may not be straight- panals this is most evident- sometimes the squares are so --- not square-- you will have a problem using them-
so look at the fabric- feel it- and decide what is ok for you and what is not

There are a lot of things that go into the quality of a fabric.

For starters, there is the thread count. Many quilters prefer the "beefier" 60x60 fabric; most Modas (not their Marbles, though), Kona cotton, Timeless Treasures, and Michael Miller Fabrics use this and some other makers do also.

Other companies print on a finer 68x68 cloth. Moda tried this with a couple of lines last year but their decision was not well-received by quilters so they are going back to the 60x60. This finer cloth was noticeably thinner and the bolts were lighter.

Then there is the quality of the printing. How delicately detailed is the design? Does it look crisp, or sort of blurry? Do the colors line up? How many different colors are used to print the design? The more colors, and the more highly detailed, the greater the cost will be. Some designs require multiple trial printings - called strikeoffs - before it turns out right.

Also there are different types of dyes used to print the fabric. I've been told the best are fiber-reactive dyes, then pigment dyes, and last is wet-printing.

Finally there's the finish on the fabric. Some fabrics are silky smooth to the touch and some are rather rough.

When you're shopping for a fabric, you're going to go for a nice look and feel. Not too thin (especially for light-colored fabrics). You will probably come to prefer certain brands. The better brands usually are quite consistent in quality though occasionally they can have an "oops!" in the manufacturing process.

The higher the thread count the better the strength, feel, etc. of the fabric.

I have notice that some quilt fabric feels different then others. How can you find out what the thread count is on a piece of fabric before you buy it.

You pretty much can't. The bolts are never labeled, and in general we (stores) can't even find out, unless we ask, or are very familiar with the manufacturer.

As a consumer, it's even harder.

You'll pretty much go by "feel". It becomes a gut-instinct thing.