I'm needing to have a 2nd water line Teed into my existing line that's running from my water well to psi tank. Will need to be inserted underground in the yard near the well head. Any plumbers have any input on how to do this or what it may cost to have a pro cone take care of it??

dresser couplings work great when you need to insert something in the middle of an existing line.

You can do it far cheaper than having a plumber out more than likely. How is your pump house built? cement floor, just a cover? Digging in and tieing to the line isn't hard if they left a bit of pipe to tie to.

No pump house, my pressure tank is in the ground. My concern is that there may be a check valve between the pump and psi tank. If I Tee in on the wrong side of the valve then it would all be for not.

you want a direct well connection or post pressure tank?

If you can see the pipe from the pump to psi tank, you will see if there is a check valve there, or just add one in the main line you splice into, or at start of the new line.

Many pumps have a built in check valve and most installers will add another in the line usually between the 1st and second pipe joint, or 20'ish on poly as a safe guard.