Eager loading fields

I have a quick question about eager loading fields. How do I know if I need to eagerly load one field? If I need to use that field in native reranking scripts, I am sure I need to preload it. But what happens if I need to query that field (e.g. fulltext), and maybe doing some reranking based on query language? Do I need to preload these fields? Also, for gps filtering or time filtering, do I need to preload these fields? or are they already preloaded?