Question: We've a Webform which picks and updates existing contact's data. Is it possible to add a "check data" step somehow after webform submission has been made but before actual contact record change? There seems to be no corresponded event-condition-action chain in Rules/Webform Rules/CiviRules.

Answer: 
Suggest you look at giving users only the option to create Draft webform submissions (maybe just hide 'submit' via js) - then you take over and do the actual Submission
Webform validation module can run a number of checks on specific fields on your webform - <https://www.drupal.org/project/webform_validation>

If you're worried about SPAM submissions install Mollom: <https://www.drupal.org/project/mollom> - and configure it to monitor your webform submissions.