Checkboxes are a funny thing in form posts. If they aren't checked then they don't exist in the form scope that the page posts to. Also, if you don't specify a value for the checkbox, and the user clicks it, then the value of the checkbox in the form scope on the action page is "on". Here is a bit of code to make sure that the checkbox always returns a 1 or 0 value.
<cfparam name="FORM.car_auth" default="0">
<cfif FORM. EQ "on">
<cfset FORM.car_auth = 1>
</cfif>
As for making sure that the other values are populated, yes you should use a cfif statement to see if the other fields need validation. Then for every field that you need validated, check if the field is defined and the value isn't an empty string (again using the cfif statement). If this conditional is true, then display validation information and a cfabort if appropriate.
If you want to stretch your brain a little (and get some cfml experience in the process), you could also make a short snippet of code that defines a comma delimited list of keys for the FORM scope. Then loop over each of the keys and use the conditional described above. I use an enhanced version of the snippet that I just described for setting up quick validation of form inputs. Instead of spending 10 minutes setting up form validation for each individual input, I use a generic snippet of code and am done and working on something else in less than a minute.
NOTE: I personally can't stand using the coldfusion provided form tags. In my opinion they are utterly horrible and are something to be avoided. But to each their own. I have seen really nice code using them.