If you're buying kits then they will often tell you what the expected Original Gravity will be. There is also not much room for variance since extract is extract.. there are no efficiency concerns or worries over incomplete starch conversion as there would be with all grain batches. The extract is just sugar.. as such the only way you can end up with a lower than expected OG would be if you didn't use all the extract, spilled it, had a boil over, or added too much top up water.
Generally speaking the final gravity of your beer will typically be somewhere in the 1.014 range though, as others have said, the sugar profile and yeast strain can have an affect on this. For example, large amounts of fermentable sugar (i.e. corn sugar) would result in a drier, more alcoholic, watery beer & a thus a lower FG.