Sample Usage:
Total Assigned Categories: {exp:entry_category_count entry_id="{entry_id}"}
{if {exp:entry_category_count entry_id="{entry_id}"} > 1}
There are multiple categories assigned to this entry
{/if}
{if {exp:entry_category_count entry_id="{entry_id}"} == 0}
There are no categories assigned to this entry
{/if}
You can hardcode in an entry ID if you like
{if {exp:entry_category_count entry_id="124"} > 1}
Your very awesome code here
{/if}