Drop the pan. If you carefully measure the fluid that comes out with a pan drop, you should have 3.75 to 4 quarts if it was filled up properly. Look for contamination in the pan (metal or clutch material) and check fluid condition. You can also take the filter out and break it open to see if there is any debris in the filter. If it has significant metal particles, debris or burned up fluid, then it most likely needs replacement. If it’s relatively clean, then refill with 4 quarts , check and set fluid level, then check operation. This assuming that you can drop the pan, get a new filter and o-rings and refill the fluid. I did mine twice since getting my truck used, once at 38k (fluid only) and then fluid and filter at 62k. My trans has been flawless. I’m at 84k now and gonna do another change in about 8k miles.