All screens are not created equal. Sorry, but there’s no way of telling what your pictures may look like on another system!
It all starts with something called “gamma” (not a dangerous sort of radiation but a measure of contrast and brightness). Most PCs are set to a gamma of “2.2” while some (especially old) Macs are set to “1.8” as a factory default.
the result is that your pictures may look much brighter or darker depending on the computer system involved.
As even modern Mac users are increasingly using the PC resolution, you are probably safe if you stick with this standard.
if you get feedback telling you that your pictures look too bright and you are sure they don’t, the other person’s computer may be an old Mac!
More about Gamma: http://www-graphics.stanford.edu/gamma.html
Even not considering the above, there may be large differences from one monitor to another model. There are many sites that offer you calibration bars for a quick adjustment of brightness and or colours.
Even better, some manufacturers even provide special profiles for their monitors. If there’s one for your model, use it!
The most luxurious option is hardware calibration: there are special devices that you put on your screen and they will generate a profile that is specific to your monitor.
Oh and as an aside - chances are that by adjusting your monitor you’ll get closer to the output of your printer.
Here are some useful sites that help you to calibrate your monitor:
it is difficult to evaluate a picture when your screen reflects the light of a lamp or is being hit by direct sunlight. Indirect light usually is best.