Depends on your laser and software.
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With my k40 I have always used an origin Jig which is made from a piece of plywood jammed into the top left corner and then cutting a square L along the axis zero lines. This gives me a precise 0,0 origin to place my work piece and uses absolute coordinates from the origin.
Like @Jammy mentioned you can have a laser pointer mounted to the head and do an outline trace if your software supports it. For engraving things that are not cut out, you can custom cut a positioning jig outline of the work piece and then lower the bed to get the right height for the engraving. I’ve also covered the workpiece in multiple layers of masking and duct tape and did a very low power/high speed outline of the object. to fine tune the positioning.
K40 Shop Tips - #8 by NedMan -
For my 50W laser, using Lightburn software, I start from where I position the head and use the attached laser pointer to outline the area to be engraved/cut to get the alignment. I also cut positioning jigs from cardboard, locked down with magnets, as needed and the lower the bed to the right height.
The Lightburn software also has a camera option that allows your to image the bed and position exactly were the engraving should go.