I am assuming you want to check gauge because you want to get an idea of how many stitches to cast on for an approximate width for your blanket. Otherwise, gauge isn’t so important for a blanket since you aren’t trying to make it “fit” perfectly.
To get an estimate, though, cast on about 25 stitches or so (I am assuming you are using a worsted weight yarn. If your yarn is heavier, you can cast on fewer stitches, if thinner, you should cast on more.) Knit for about 20 rows or until you have a long enough piece that you can get a good measure of the width. (Usually a gauge swatch is around 4 inches by 4 inches, but in this case, you don’t need as much because you are really just trying to get an estimate.)
Bind off the swatch, lay it flat, being careful not to stretch the stitches (you can pin it down if you want), and lay a ruler across the center of it to figure out how many stitches there are in one inch. Then multiply that number of stitches by the number of inches wide you want your blanket to be and that’s the number you cast on.