How do I check my iPhone for spyware/malware

You don't.

There is no known malware that can affect an iPhone.

There's no way to install spyware, unless someone had physical access to your unlicked iPhone with a computer for an extended period of time.

There are no apps that check for these things, since due to the nature of iOS, there's no way for them to scan outside of the their own designated app area. iOS is sandboxes, that means apps cannot access areas outside of their own app sandbox.

If you explain what issue you are experiencing, someone may be able to an advise, but it's almost certainly not malware or spyware.