1953 Mantle Bowman Color
 

Dad found this Mantle amongst a box of 52's-58's. It looks too good to have been just placed with the others. Measurements are correct, but how can I know if the corners have been altered. (yes I read the article linked above) Any help or thoughts would be appreciated. Vertical and horizonal sides appear to be the same consistency.

Welcome to the forum. From the scan the card doesn't look altered to me. The best way to get it authenticated would be to send it to one of the big 3 grading companies. The cost will be somewhat minimal ($20sh) and then you will have a professional opinion, that while isn't perfect, is marketable if you go to sell it. Good luck, nice card. Also, if you keep posting in our forum please know there is a section for Post-WWII cards. This section is mainly for Pre-WWII cards. Take care...

Looking at the corners, the question of alteration doesn't enter my mind. They simply appear to be a bit dented, which is very common. I agree that it would be worth your while to get it graded if you're looking to sell it.