Wade,
I check for evidence of septic system failure as required by VC 2A-you know the stuff to look for I'm sure:Grass is greener over the septic tank, smelly odors, soggy yard by the septic field, when you flush the toilets on the lower levels, they empty poorly or back-up,etc. Also, if you are in an area of known septic system failure such as neighborhoods with high water tables, lakes, etc call for the cert using that as the reason for the cert. I still remember the one I did years ago where the septic system field was at the same grade level as the water filled detention pond behind the subject-sure that system is going to drain properly-NOT.
Of course, as Ron said, you check VC 4A, if no problems are observed and let the DEU worry about it from there.
As Pam stated, a statement that you are not a septic system specialist and that a test/cert is recommended could never hurt either.
One FHA "trick" to know-If the home is vacant when you inspect, ALWAYS call for a septic system certification. If it's vacant, even with the utilities operational, you can not observe the septic system for proper operation because, well, no one is using it and subjecting it to a daily load-no pun intended there. Do not automatically assume the system is OK. I've seen situations where people will vacate a home with a bad system which will pass our silly, run-the-water, flush the toilet tests because all we're doing is filling the plumbing lines with a small amount of water. So if it's vacant, state that you can not observe the system for proper operation and call for the certification. The same goes for snow covered sites-if you can't observe it (the septic system field), state why (the snow), then call for the certification.
Remember, think of a reasonable reason (I like that, reasonable reason) to pass the liability onto an expert. If a major, costly item such as a septic system fails as soon as the new buyers move in, you can be sure that a complaint will be filed against you with FHA (buyers never make mistakes, right???) and you will be rewarded with a field review in response to the complaint-don't need that. Not to mention, a lawsuit.
Ben