Use a LM2917 frequency to voltage converter.
To actually obtain the pulses, I have used both a photo reflective detector or a hall cell.
On the former, paint a pair of white stripes on opposite sides of the flywheel. Unless you have very strong ambient light, this would be the preferred method.
Otherwise, use the hall cell method. Glue a pair of small magnets on opposite sides of the flywheel. Make sure they are properly aligned and are identical in weight to avoid unbalancing the flywheel. Also the glue should be a strong two-part epoxy, unless you want magnets flying off.