Then use the scope to figure out the data format, and subsequently the PC to read all data and look for relevant content.
For the first steps, you don't need serial decoding at all: Check the "idle" signal level (high or low); check the minimum duration of a bit to figure out the baud rate.
If you want to endable decoding on the Rigol, you can do that, and it does work on a stored trace -- decoding essentially happens during the screen display rendering, if I recall correctly. You will need to tell the scope which channel to decode, and what the signal polarity, baud rate etc. are. So it is not more "automatic" than the decoding your PC provides, and only a little more flexible (the signal polarity can be chosen). Since decoding does work from the screen contents, the Rigol scope can only decode bytes which are fully displayed on the screen, from start bit to end.