If you know how long a character is supposed to take at the baud rate at which you're operating, then, having sent an AT, which should produce an OK, you can time the length of the string.

Starting with the baud rates in the order of your own preference is not a bad idea, i.e. send AT and listen for OK. It's only if that doesn't work that you have to do something more involved, like sending your AT command and then "listening" for the start bit and timing out how long the response is. It's likely to complain in some way, typically with "ERROR" or some such. If you know what the modem is supposed to say when there's a framing error, you should be able to tell from the length of the predetermined response, what, approximately, the baud rate is. Async comm's are not that precise. They're usually happy with ~2-3 percent.

If you "try" the baud rates in the order of your own preference, you'll likely end up with one you like.

RE