Sometimes I use "re-positionable correction tape" -- Post-It tape -- to mark a seam allowance. Not often, because it happens that the right edge of my machine's needle plate is exactly half an inch from the leftmost needle position -- rather odd, when the machine is metric
-- and half an inch is the seam allowance I use most often.
For narrower allowances, I guide on the feed dogs and various parts of various feet, and shift the needle positions.
When I'm hand sewing, I use correction tape on the fabric. Since it tends to break loose when the fabric flexes or stretches, I use an inch or less and keep shifting it to mark my way.
If I need to mark fabric for machine sewing, I usually use a water-erasable marker, but I also have an assortment of pencils -- the most-useful pencil isn't an official sewing marker, but one with a "lead" of real calcium-carbonate chalk, the sort that was used on blackboards in the first half of the twentieth century.
(Modern blackboard crayons are not chalk and might stain fabric. Real chalk is a cleaning agent, and quite safe for color-fast plant fibers.)
Joy Beeson