Klugesmith wrote ...

You can determine the wavelength by measuring the diffraction pattern with an inexpensive (and may be unusually coarse) grating between laser and target surface. If the grating pitch isn't exactly known, you can use a laser with known wavelength as a reference.

A fine wire screen (such as used in fuel filters) can serve as a grating and not, I expect, be burned up by the 1W laser. I can send you a bit of suitable screening.

With other grating materials, you may need to turn down the power or use some optics to spread the beam without overly expanding the spot on target surface.

A source of free gratings for demonstrations is the linear encoder ribbon found in many discarded inkjet printers.
In USA they are typically 150 lines per inch, but you can trust it's a round number and can directly measure it with some care. For example, juxtapose it with a reference scale & make enlarged photograph or photocopy on which you can count the lines.
Another source, of course, is CD's or other optical disks. You can look up the standard pitch, and/or use a wavelength reference laser.