On the Marriott Website, when you look at properties to book, you can either look at different rates (Marriott Best Price, AAA, Government, Senior, Corpoate, etc) or you can check the box to use Rewards.
When looking at properties with the Rewards option checked, it will tell you how many Marriott Rewards points are needed for the level of property (1-8), and duration of stay. It also provides the Marriott best price, as you can opt to use a comination of paid nights and reward nights. It also shows the property level, and if its a cat 1-4, and you have any certificates, these will be applied prior to using points.
When you select the property to book, this is when it shows you how many points you have. If you are staying more than 1 nite and have insufficient points for all nites, then you can adjust paided and points (as long as you have enough points for 1 night). If it has applied a Cat 1-4 certificate, it will also let you know.
You can also see your points on the, "My Account" section on the upper right hand section of the home page. The 2nd entry from the bottom, is the, "My Rewards Activity" link where you can see your balance, detailed line items of your earnings and uses, and you can even drill into a stay, and see a breakout on the stay points, including base points for the stay and expenditures, bonus points for your reward level (i.e. Platinum is 50%), gift points (Full Service Marriott and Resors are 500, and FFI/RI/CY/SHS are 200)), etc.
Or you can just call customer support and ask.