You can check exposure with the histogram. There are lots of tutorials on how to do this.
As for how it looks in the end, you need to check the image on the medium you will use.
I would suggest printing a photo and then comparing it to your monitor under good lighting. A desk lamp with a daylight bulb would work. Simply adjust your monitor until it looks similar to the print.
The issue with sharing images online or on facebook is that everyone has different monitors with different settings. I like to use my iPhone as a test since it only has a brightness adjustment, so its like a happy medium.
There are monitor calibration charts, but they can be tricky to use and won't give you perfect results. There are monitor calibrators, but they are pretty expensive. I've used the print method for a while now and I've been happy with the results.