Transmission Oil

I did a big synthetic swap this summer, put new stuff in both my diffs, T-case, engine, and my antifreeze and brake fluid are still fine.
But i'm a little worried about my transmission, i've looked at the oil and it looks like its still clear, but I bought the truck used at 29 000 miles and its about to hit 50 000. How should I know when this oil needs to be changed?

Any advice? Thanks in advance