If your chain is sharp it should cut well with very little force on the bar. If you have to force the saw to cut, the chain is dull and needs to be sharpened.
A visual clue that it is time to sharpen your blade is if the chain is spitting out dust rather than just chips.
In addition to being not being as efficient as a sharp chain [not to mention causing unnecessary wear and tear to your chainsaw], a dull chain is very dangerous because it increases the chance that it will kickback– so be sure to keep your chain sharp and be safe.
For tips on how to sharpen your blade go here.