I am from Australia, so don't know how relevant my answer will be for the US, but I'll try anyway.
If you are having your utilities (electricity, gas and water usage) included as a set fee as part of the rent, I would say that the landlord has taken the average cost for these utilities and added a buffer incase you and your house-mate use more than the average. They would probably then review the costs after a certain period to see if they need to incease them or not.
If you think you are a low user of the utilities (ie use water sparingly, turn off the lights when you leave a room, etc.) then you may be better off paying for the utilities separetly. However, since you are sharing with someone else, and if you don't know whether they are a high user or low user, then you may end up paying for some of their usage as well. These are the things you may wish to consider.
In the end you will have to decide if you want to pay one single bill each week or month, or have all your costs separated so you know you are not over paying for something you may not be using. Some people feel that not knowing and making one single payment is easier and less stressful, while others may be concerned about not knowing and need to know all the amounts to put their minds at ease. You have to decide which type of person you are.
Personally, I would be the type that wanted to know the exact amounts to pay, but as mentioned earlier, this is made harder by the fact that you will be sharing with someone else.
To estimate the amount for each utility you will need to firstly estimate what your usage is for each utility and then multiply this by the rates charged by each utility company (this can be found on one of the utility company's previous bills). Maybe you can ask the Landlord that you wish to pay the utilities separately for the first couple of months and then decide if you wnated them included with the total rent after that. That way you can work out actual costs and decide which outcome is better for you. There is no harm in asking.