If someone untrustworthy has had physical access to your phone, it is safest to simply assume that it has been compromised. Restore the phone from a backup made prior to the incident and change any account passwords. If you have sensitive personal information - such as credit card numbers, social security numbers, bank account numbers, etc - in unencrypted form on your phone, those should be assumed to have been compromised as well and monitored carefully. (And, in the future, don't put this kind of information on your phone unless it is locked up in an encrypted file of some kind, such as in an app like 1Password.)
As to whether you can determine whether the phone was actually accessed... I think you're chasing your tail on that one. It certainly may be possible that the failed lockscreen login attempts are logged somewhere, but if so, what those logs might tell you doesn't necessarily mean anything. For example, with physical access, someone could connect the phone to a computer and use special software to download data, without unlocking the phone.