Why do you have to give the number to AdSense? May be you're talking about Google AdWords.
Anyways, you can check the security of the website by looking whether the url starts from http or https. If it is https, then it is secure. But Caution: AdSense will never ask your MasterCard number. I think it is Adwords. Also, do not give away your card numbers to anyone via e-mail. It should be an automated website starting with https.