Is there a way to see what fields are used in Rules?
Consider a rule as in this (random) sample (in Rules export format):

{ "rules_new_node_with_special_optionlist_value" : {
    "LABEL" : "New node with special optionlist value",
    "PLUGIN" : "reaction rule",
    "ACTIVE" : false,
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "ON" : { "node_insert" : [] },
    "IF" : [
      { "entity_has_field" : { "entity" : [ "node" ], "field" : "field_optionlist" } },
      { "data_is" : { "data" : [ "node:field-optionlist" ], "value" : "C" } }
    ],
    "DO" : [
      { "drupal_message" : { "message" : "A new node got created with option list value =  \u0022[node:field-optionlist]\u0022" } }
    ]
  }
}


Obviously, the above rule uses a field with machine name field_optionlist.

Execute an SQL query on the Drupal database which looks like so:

SELECT id,plugin,active,name,label
FROM `rules_config`
WHERE `data` LIKE '%optionlist%'
LIMIT 3


The results I get by using this query (limited to the first 3 results only) looks like so:

id  plugin         active  name                                           label

20  reaction rule  0       rules_verify_updates_to_node_properties        Verify updates to node properties
21  reaction rule  0       rules_new_node_with_special_optionlist_value   New node with special optionlist value
28  rule           1       rules_set_an_initial_value_for_a_text_field    Set an initial value for a text field


Obviously, the result with id=21 matches with my (random) sample rule.

To execute the above query, you can use whatever SQL query tool, such as PhpMyAdmin. Or if you rather want to have a site building tool, just use the Forena module (disclosure: I'm a co-maintainer). After you enable this module, these are the steps required to find the answer to your question:


Use relative link /admin/structure/forena/data/add (= Create New SQL Query).
Select a "Data source" in which the SQL statement (= data block) will be stored (saved).
Enter the above SQL statement, and use the Test Changes button (to prototype your SQL).
Voilà, there is the (prototyping) result.


The above should answer the "It would be enough to check if a certain field is used by Rules" part of your question, but if you want to further improve this prototype, use the power of Forena to build a custom report from it, and make all sort of magic happen, eg:


export it to a CSV.
eMail it to somewhere.
...


Note: the above approach works for all sorts of "Rules queries" ... after all, in the above query I used (part of) the machine name of some field (= optionlist). Do an educated guess what kind of results you get if you would use the machine name of:


a content type, e.g: article.
a rules component, e.g: rules_send_email_notification.
a module that some rule(s) depend on, e.g: webform_rules.