How can I tell what model Surface Pro I have?

The serial number of your Surface is printed directly beneath the word Surface under the kickstand on the back.  Once you have the serial number, enter it here: https://mybusinessservice.surface.com/en-US/CheckWarranty/CheckWarranty and choose Submit.  It will display the model number for you.

An alternative is to look it up on the device itself.

1. Using Cortana, search on system information.
2. Select System Information in the list of results.
3. Find System Model under the Item column.
4. The model info for your Surface will appear in the Value column next to System Model.

Here are a couple images to show you where on the tablet itself you can locate the serial number.