All you need to do is open the file using an image viewing program, such as Preview or GIMP. If the image looks distorted or otherwise incorrect, then it is likely that the file has been corrupted. Additionally, you can use online tools such as PNGGauntlet and PNGChecker to check for errors in the file's compression and structure. Finally, if all else fails, you can compare the size of your original file to its compressed version - a significant difference in size would indicate that there are issues with the file itself.