Highlighting is enabled on all searchable attributes by default, unless specified otherwise in attributesToHighlight.
If you are setting these in a library like InstantSearch.js you can refer to the relevant documentation here on how to send those attributes.
If you are having trouble confirming whether these are being sent, see our guide on verifying the search parameters sent.
There is more information available here - https://www.algolia.com/doc/guides/building-search-ui/ui-and-ux-patterns/highlighting-snippeting/js/#highlighting