How can I know if my iPhone has malware?

The best option is to start it as a new iPhone without using backups, best would be to create a new Apple ID. After having said that, if you didn't jailbreak your iPhone it's not possible to get malware/virus/spyware, an iPhone doesn't need any antivirus, and the most common scam is phishing sometimes confused with a malware in the phone, that has got nothing to do with it

Recognise and avoid phishing messages, phony support calls and other scams – Apple Support

It's not clear what you mean by "6 card jacks in 6 weeks". Do you mean that you've had credit cards compromised? If so, that is not something that could have been caused by anything on your phone, even if it had been hacked. However, KiltedTim is correct. Unless you have jailbroken your phone, it hasn't been hacked.

Instead, you should think about where it is you've been using the cards. Any dodgy ATMs? Your card issuers should be able to give you some information.

Do NOT create a new Apple ID. That will not solve your problem and will just create all sorts of other issues.

If you did not jailbreak it, you do not have malware.