Check what methods were used on element query
Most of the time, you can check if an ElementQuery parameter has been set by checking the corresponding property name: it will be null if the parameter hasn't been set, and have the previously set value otherwise.
Examples
This will output 6:
  {% set query = craft.entries().relatedTo(6) %}
  {{ dump(query.relatedTo) }}

This will output null:
  {% set query = craft.entries() %}
  {{ dump(query.relatedTo) }}