/cast [help] Greater Heal
The above will show the tooltip for Greater Heal when you mouse over it.
Alternatively, you can specificy the spell name and use the same type of conditionals in the showtooltip line that you can use in the actual macro.
If you did not include the "Sap" part next to the showtooltip, it would have shown shadowstep's tooltip.
This is especially useful for cast sequences and conditional macros.
/cast [nomod] Blessing of Kings
/cast [mod] Blessing of Might
Will show bok or bom icon/tooltip depending upon whether a modifier is pressed.