Hi Patrick, you need to contact MTN support, they will help you to check when the card was used, they will not tell you the number that used it unless if you provide to them the number you are suspecting for them to confirm. You can also report the issue to the MTN Trade partner you are buying the card from, they can assist you.
You can contact MTN support via any of these channels:
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001