I use my seam gauges all the time (have several as I tend to lose the slide bit, or bend the gauge or otherwise destroy them), they make checking seam allowances, hems, spaces between buttonholes etc etc very quick and easy.
Oh and the Fray Check - I squeeze a blob onto a thick piece of spare paper, dip a cocktail stick into the blob and very carefully drag it along the inside of the buttonhole, thus avoiding the drippy blobs and ruining the garment. (this is a tip from Louise Cutting of Cutting Line Designs). Works a treat.