how to validate if certain fields are filled and if some are not filled

how to validate if certain fields are filled and if some are not filled, flag.

You can set up a field as required to validate that the field is filled.

Apart from that, you have Form warnings to validate the field entry - https://www.jotform.com/help/61-How-to-Change-Form-Warnings 

Flagging can also be done on a form submission but not on a field per se on your form Submission page.

Do let us know how you want to do the checks so that we could guide you accordingly.

So, when you click the Submit button, check if certain fields are filled in, and if they are not, display a message and do not proceed until the fields are filled out.

To verify the filled fields, before submitting form, you can use the Preview Before Submit widget on the form.

If you mark fields as required, then the form will automatically show the required field error message. So users will not be able to submit form unless the required fields are filled.

Related Guide for reference: https://www.jotform.com/help/546-How-to-Set-a-Field-as-Required-or-Non-Required 

Do review and let us know if this helps.