...a trained industry professional to physically open one up and have the knowledge to estimate it's condition.
You will of course want a home inspector to go over the home for you, but the Home Inspector is not a trained septic industry professional. The home inspector might pour a colored dye down the drain, turn on all the faucets in the house, and run a lot of water through the system to see if colored wet spots appear somewhere in the yard.
The Home Inspector is not conducting an adequate septic system assessment--the dye check is only a weak test. You need to look INSIDE to see what it looks like, AND know all the clues you are looking for. The tank should be opened and have a functional assessment made. Hire a local septic maintenance company to do this for you. They will be able to give a much better understanding of the condition of the septic you are considering purchasing.
First hand experience: The sellers of this home previously were New York State real estate agents (for 10 years), and over that time had many opportunities to give buyers this advise, and on several occasions the results were similar to this house. Two were outright failures, one was described as a "septic" in a well respected listing agent's marketing materials that turned out to only be a cesspool (no longer a legal installation). Another was a tragically comical harry-homeowner modification to keep the system "functional" (a big-time yuck) while the vacant home was being shown for the sale, Several other home's systems were questionable but had no additional yard or property on which to build a new one.
Chances are, when you get a local professional they may have prior knowledge of the system if they have been called out in the past to pump it...That is what happened with this house when the sellers bought it in 2005. The guy showed up and the first thing he said was "I've been here a few times!"
He didn't even have to open it up to know that it was the original septic, built back when building codes for septics were just a step up from outhouse level systems. He had already been here and knew what this system was made of because he knew all of local building styles over time since septic systems first went into use. When you do this stuff as a professional for a living you see everything. Everything. Giving you a valued opinion is the fifteen minutes of fame for the septic guy. He knew that this one had a very small drain-field and the pine tree planted to the rear of the home had grown it's roots through the drain-field pipes to clog things up. Not to mention that the number of people living in the home for several years had been more than the system was designed for (using present day building codes as the guide). The system had already failed prior to the home going on the market, and the septic guy knew this.
It might cost you an extra couple hundred dollars for this service, and you should pay it.
You won't need to do any inspections on this system, but you can if you want to. The sellers will give you the drawings and photos of the installation. They have been living with septics for the past 28 years between Los Angeles and here in Warwick, and have never had to have one pumped out.
Fortunately this house came with a very large side yard slightly down in level from the house, where a new system built to modern building codes could be installed. Many old houses do not have this 'luxury.'
This home's system was built for an additional bedroom in case anyone in the future wants to expand their household without moving to a larger home. Unless building codes change, there will be no need to do an expensive expansion of the system. It is a four bedroom system on a three bedroom house (systems are designed for number of bedrooms, not bathrooms). Finally, for the entire time since the system was built, one bedroom was being used full time and another bedroom was being used one weekend a month. The load on the system for all of the nine years of it's life thus far has been exceedingly light.
What the photos are trying to show...The two photos show how the new system fills the side yard, where the tank is in relation to the house, and that there is a line going up to the red cottage/barn--in case anyone ever wants to re-build that thing into a dwelling or something. The second photo shows where the drain-field ends and that there is room to drive a car from the garage/cinderblock barn around the log barn and in front of the trees over to the rear of the house without running wheels directly over the drain-field.