On my 92 VX I use the MPGunio and the vacuum gauge to tell me I'm in lean burn and also how close I am to falling out of lean burn.
Here is how I have the MPGunio and the vacuum gauge located up on my dash. They are the two gauges I look at the most in order to maximize MPG
When I am driving I have the "Guido" set on the instant MPG screen (it is the screen next to the one in the picture, sorry my bad.)
This screen tells me my current instant MPG and if it suddenly shows my MPG has dropped in half, like from 50 MPG to 25 MPG while I am under load climbing a hill then I know I am out of lean burn and I will need to back off the throttle a little to get it back to 50MPG and back into lead burn.
You can also "feel" a surge in engine power as you leave lean burn, but that "feeling" is not always noticeable enough to be reliable, hence the need for the gauges.
If you want some advanced notice before you are about to go out of lean burn, the vacuum gauge will tell you that.
At light load cruising or near idle the Vacuum gauge will be in the range of 22 to 15 inches of vacuum, but as you accelerate the inches will drop toward 0 at wide open throttle.
The computer in the VX will drop you out of lean burn somewhere between 5 and 3 inches of vacuum, depending on how accurate your vacuum gauge is. It doses this because it senses you want more power and lead burn is not the max power of the engine .
So, if you watch both the instant MPG line on the "Guido" and the position of the vacuum gauge needle, you can decide if you want to make throttle adjustments to stay in lean burn,
When I am starting from a complete stop and shifting through 1st, 2nd, and 3rd gear, I cannot stay in lean burn. also, if you are climbing a hill on the freeway and need to keep up with traffic, you will probably need to exit lean burn to get more power from the engine. These are the only times I am unable to stay in lean burn on my 54 mile commute each day.
Let me know if I didn't make anything clear.