Parse for the following commands:

/cast (check conditionals to determine which spell is being cast if any)
/use (same as /cast)
/castsequence (check conditionals and then query the status of the castsequence)
/stopmacro (check conditionals and if there are none or they evaluate to true then stop checking for icons)

When determining the icon from a spell, check for Spells, Items and Toys.

Store the above operations in order in a per-macro cache to avoid reparsing the actual code. Make sure cache is reset when icon is updated.