Within Photoshop, simply open up your image and go to IMAGE...IMAGE SIZE and the dialog box will tell you the resolution/pixels as well as the DPI.
Within Windows, you can simply view the folder that contains your images and change the view type to DETAILS and it should tell you the resolution of your files in one of the information columns.
If you are unable to determine the resolution of your images, ArtWanted.com will tell you the resolution when your files are uploaded. We will also calculate automatically which print sizes you can offer from the file you have uploaded.