Gerald wrote:
Check with Alaimo Group Mount Holly, (609)267-8310.

You have a very difficult problem indeed.
Nobody on the Internet will likely be able to help you.
I have a similar problem in Lake Hopatcong, NJ.
I called a dozen plumbers in the local yellow pages.
They all basically said the same answer.
FWIW, here is what I learned about the problem & the solution.

THE COST:
- The pipe-inspection people charge about 2000 dollars a day
- My application, they said, would cost about $1000 to inspect (1/2
day)
THE PROBLEM:
- I have a Z-shaped 10" diameter cast-iron stream outlet pipe
- The inlet is about 7 feet above the outlet
- The distance from the inlet to the outlet is about 10 feet
TWO PIPE TESTS ARE AVAILABLE:
- Place a FERNCO cap on the outlet & measure leakage from inlet
- Place a EXPANDABLE BALL on the outlet & measure leakage from inlet
- Send a VIDEO CAMERA from the outlet to the inlet & inspect for damage
FERNCO CAP:
- The Fernco cap sits on the end of the exposed outlet pipe
- It is clamped down (requiring about 2 inches of pipe to grip)
- Over a 24-hour period, measure leakage from the inlet area
BALL METHOD:
- The ball method entails plugging the outlet 24 hours
- The ball expands with compressed air to block the outlet
- If the inlet area shows signs of drainage, then there is a leak
VIDEO METHOD:
- The video method entails sending a light & camera up the pipe
- The video camera allows the operator to see any cracks & clogs
- 45 degree bends are acceptable (90 degree bends are not)

I was able, after much searching in the yellow pages under "plumbers"
and a day of searching on the Internet for "pipe inspection drainage
sewer" to find only three outfits that perform this inspection task in
the whole of North Jersey. None of them could send a camera up because
of my 90 degree bends. None could use the FERNCO CAP because the outlet
was flush with a rock wall (therefore there was no 2 inch grip). All
could use the ball method, which is imprecise at best.

For my $1000 dollars, I'll just have to build a new outlet pipe for my
stream (unless some genius on the Internet has an idea which actually
works).