I am not aware of anything like that, no. The problem is what you are seeing as characters are a complete corruption of the data. Only some part of it may have been interpreted as a valid bit sequence, and even then you can't know what bit it was.
However, there is one magic character in UART communication:
What is special about that character is its binary pattern. It is ASCII character 85, which in HEX is 0x55, or binary it's 0b01010101. Add to that the start and stop bits, and invert it for the UART logic (1 = LOW, 0 = HIGH) and the signal that gets sent out of the UART looks like:
____start_____ _____ _____ _____ _____
| | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 | stop
----- ----- ----- ----- -----
And yes, that's a pretty good square wave. The frequency of that square wave will be exactly half the baud rate - so a frequency of 4800Hz will be a baud rate of 9600 baud.
This effect is cleverly used for a system called Automatic baud rate detection whereby a receiver waits for a
U character to determine the baud rate to use. Most modern microcontrollers have this facility available.
If you can't get the frequency of a block of
U characters you will have to examine the waveform with an oscilloscope. It should be possible to identify the individual characters in the data stream as distinct blocks in the waveform. From that you can then work out the width of one symbol and thus the baud rate. It's best to use a DSO for this since you can then capture a short burst and use the markers to measure the time between different points in the waveform. A logic analyser will also do the job quite well, and will most likely be able to decode the waveform and calculate the baud rate for you.
If you have neither of those it would be possible to program an Arduino to capture the time between transitions of an incoming signal and allow you to plot them in Excel to give you a waveform you can examine to determine the baud.
Another option is just to keep trying different baud rates (there's relatively few "standard" baud rates) until you see the right text coming in.