You can not check your image resolution once it is imported into Canva, and Canva does not provide any resolution warnings. If your imported images appear pixelated (like the image below does), then you should attempt to find a new image that is our recommended image resolution of 300 dpi.
You cannot check your document resolution until you download it as a PDF. Your document resolution after exporting it for print will be 300 dpi, which is our recommended file resolution.
Please refer our File Saving Guide for Canva here.