Looks like it would be across the top of that picture, as mentioned probably with a plug on one end or the other. The adjustable ones usually have a exposed nut with a locknut. Yours may be factory set using shim washers. If there is a plug on one end or the other, of the red valve body at the top of your pic, try removing it and see what is underneath. You will probably find a washer or two, a spring and a valve seat. The tension on this spring determines at what pressure the seat opens and starts to relieve pressure. If the plug sets the tension, then their will probably be some form of setscrew along the valve body that holds it at it's set point.
To check the hydraulic safety release pressure, buy a hydraulic pressure gauge with the same type fitting that your hoses on the working ports(hoses running out to cylinders) of the valve use. If you are expecting 2000 PSI, buy a 0-3000 gauge. Remove one of the hoses and screw the gauge into the port where the hose was. Before removing the hose, Make sure the loader is solidly on the ground, engine is off and no pressure is on any of the lines by working the valve to all it's positions. And of course when working with hydraulics, always wear eye protection. A jet of high pressure fluid can ruin an eyeball in short order. Place the hose you disconnect into a bucket and secure it there. This is in case you send fluid to the wrong port. If you do this a cylinder will move and the fluid forced back from the other side of the cylinder will flow out of the hose and into the bucket. Start up the tractor and try and send fluid to the port you attached the gauge to. Since the fluid has no where to go(dead headed into the gauge), you should quickly reach the safety valve opening pressure and get a wooshing sound as the fluid flows thru the safety. The pressure reading on the gauge while this is happening is the set pressure of the safety valve. Any adjustments you make can be tested in this fashion and when completed, the gauge can be removed and the hose re-attached to the working port.
If you want a permanent gauge, you will have to add a "T" onto the fluid input line into the valve and put the gauge onto the open port on the "T". With this in place, you can check the safety pressure anytime by just running a cylinder to it's end stop(full up, full down, full curl or full dump). When the cylinders reach their end stop, the safety will open and the relief pressure will be displayed. A permanent gauge also can help you estimate just how much load you are lifting.
If your pump is working OK, you should still be able to reach full pressure at lower engine RPM's, it will just take a little longer to do so as the lower RPM means it is moving less volume per minute.
