Best way to check if a field exist in an Elasticsearch document

You can use the exists filter combined with a bool/must filter like this:

{
  "query": {
    "filtered": {
      "filter": {
        "bool": {
          "must": [
            {
              "exists": {
                "field": "price"
              }
            },
            ...     <-- your other constraints, if any
          ]
        }
      }
    }
  }
}

DEPRECATED (since ES5) You can also use the missing filter combined with a bool/must_not filter:

{
  "query": {
    "filtered": {
      "filter": {
        "bool": {
          "must_not": [
            {
              "missing": {
                "field": "price"
              }
            }
          ]
        }
      }
    }
  }
}