Gmail provides Hijack detection tool
It's not 'Eureka'! I admit...
I know this is not any extraordinary feature and somebody might be familiar with it. But this post is for unfamiliar ones and I was amongst them until I searched this feature.
I don't say this one is going to provide full proof security, but yes, to some extent you can make yourself sure that your account is not opened at any other machine except you, what activities have been performed and at what time, form which IP etc--every bit of information can make yourself more confident.
The feature can be found below the user interaction area.