Our toilets are starting to flush slow and we have a wet soggy spot in the yard where our leechfield is. How do you know if you have a failing system? Robert

If you have these obvious signs, sewage surfacing in the yard and/or backing up into the house, then you have a failing system.  This is not real tough to figure out.

There are 3 main reasons that a properly designed/installed system fails:

Hydraulic overloading-In other words, you put more water down the drain than it can handle. A septic system can only handle a certain amount of water per day. For a 3 bedroom house the system should be designed to handle a maximum of 450 gallons of water per day. It doesn’t take a rocket scientist to figure it out that if you are putting 600 gallons of water into that system every day something is going to go wrong. The obvious solution is to stop using so much water. Note: although this can be termed a temporary failure and easily correctable by reducing water consumption, when you are using water at this rate it will cause other long term damage. First, this water that is going into the system at a high velocity will not allow the solids to settle in the tank (as is the purpose of the tank) and will get flushed out to the drainfield (adding to cause number 3 of failures). And second, this water will agitate the existing contents of the tank and will flush more solids out to the drainfield.
Chemical overloading-Harsh chemicals like automatic toilet bowl cleaners kill bacteria in the toilet, but that killing process does not end there…it continues through out the system. Eventually the good bacteria in the tank and soil will be killed off as well causing a failure. Note: although this can be termed a temporary failure and easily correctable by reducing chemical usage, when the bacterial action stops in the tank, the solids stop breaking down and can easily get flushed out to the drainfield (adding to cause number 3 of failures).
Solid intrusion in the drainfield- solids that are supposed to remain in the tank get flushed out to the drainfield and plug the pores of the soil and will no longer allow the liquid to flow through it.  This is what is termed a permanent failure.  
Think of your septic system as a 2-stage filter.  The tank is the first part of the filter: as wastewater enters the tank, the solid materials contained in this wastewater settle in the tank.  The liquid moves on to the drainfield where naturally occurring bacteria in the trenches and soil surrounding the trenches then "eat" the parasites and viruses present in this liquid. 

The problem is, the liquid leaving the tank is not crystal clear...it contains tiny solids and these solids will begin plugging the drainfield and soil.  Some of these solids will break down in the field (feces, toilet paper), but if you get enough of them out there they impede the draining process...the water stays in the field area longer and will drown the aerobic bacteria.  Once the aerobic bacteria are gone (leaving only anaerobic bacteria which are 20 times less efficient than aerobic bacteria) the system goes into failure.  What you have is essentially a dead pool of water because now the cleaning solvents build up in the field killing off the rest of the bacteria. 

Now if you pump the tank and stop using the system for a few years, the field will drain, the aerobic bacterial colonies will repopulate, the organic solids will breakdown and the system will start working again.  But there are 2 problems with this.  First, not many people can go 2 years without using water and more importantly, many of these solids are non-biodegradable and will never breakdown in the drainfield. 

The leading source of non-biodegradable solids is washing machine discharge.  Washing machines discharge a tremendous amount of tiny fibers (lint) that do not have the necessary mass to settle in the tank...it is like a powder and gets flushed right through the tank out to the drainfield.  Most of our clothing and carpeting is manufactured with polyester and nylon (we live in a wash and wear society).  BACTERIA DO NOT EAT PLASTIC!!!  There are cases of huge, hi-tech Laundromat septic systems failing in less than 2 weeks.  This shows the damage lint will do to a system.

Other sources of these non-biodegradable are "wet strength" paper towels, cigarette butts, feminine hygiene products, and condoms. 

The way to minimize these solids from getting out to the drainfield and plugging it up are:

Pump the tank on a regular schedule.  As the solids build up in the tank, they take up space...the less space there is in the tank the less space there is for settling of the solids.  If you let those solids build up too far, when you put solids in one end, it will push solids out the other end...right into the drainfield.  
Refrain from using large volumes of water for prolonged periods.  When mom does 7 loads of laundry on Saturday morning along with 5 people taking showers, the water is flowing through that tank/system like a machine gun.  The solids in that effluent will not have an opportunity to settle out and the existing solids in the tank can be stirred up causing them to be flushed into the field as well.     
Install filters-Effluent filters go in the tank and stop the larger solids from getting out to the field.  Washing machine filters (these attach to the washing machine discharge hose) will keep the fine partials out of the system.  USING FILTERS IS THE BEST METHOD OF KEEPING SOLIDS OUT OF THE DRAINFIELD!!!