you put your needle down then use a ruler and put it against the needle; make a mark at the 1/4" line then you can put a piece of tape on your machine as a guide.
a good way to learn where your 1/4" is; is to cut 3 strips of fabric each 1 1/2" wide by about 6" long; sew them together with what you think is 1/4" seam, open, press your rectangle and measure it should measure 6" x 3 1/2" if it doesn't you adjust your seam allowance in what ever direction you need to (wider or narrower) and try again until you have it consistant.