It depends on what software you are using to transfer photos from the camera to your computer.
If you install and use the "Camera Window" transfer software Canon included with the camera, you will see you have three modes to choose from, including the option to transfer just the photos on your camera that have not yet been transferred.
Connect the camera with a USB cable to your computer.
Power up the camera.
The computer will detect the camera, and open a dialog box offering you a choice of programs on your computer that can handle photo transfers.
If you choose the Camera Window software, it will open and present a menu with three choices of action. The first option is, "Import images from Camera." Select this.
Another submenu opens presenting these three options:
1. Import Untransferred images. (this is what you said you want)
2. Select images to Import. (manually select just certain ones)
3. Import All images (automatically transfers every image on the card)
Menu screenshot, Canon's "Camera Window" transfer software
Camera Window is just one of many available image transfer applications you could use. They all do essentially the same three things outlined above, including "Import Untransferred images."
Canon included a couple other utility programs on the CD that came with your Powershot A490 camera. One of them, "Zoom Broswer EX" enables you to reload photos from your computer, back on to your camera's image card, if you want to do that.