Know who has access to your Google account information
To check out what sites and apps have access to your Google Account, just use this security permissions link. Opening it should give you a screen that looks something like this:
As you can see in the above screenshot, there are various types of account authorization through these services. Full access allows complete usage and modification of your Google account. That means full access apps can read your mails and contacts, track your activity and change your profile information. And therefore, it is essential that you give full access to apps you completely trust. In my case, it is Chrome and Ubuntu, which are my primary browser and Operating System that can make full utilization of my Google account. Other accounts have “some” access, or access to particular Google services. This means they can’t directly change or interact with your Google account or its credentials, but they can use parts of your account information. Google+ and account info, for instance, allows users to quickly create accounts on supported websites. Take a look through this list. If there’s a site you don’t use or recognize occupying it and having access to things you don’t want it to, just remove it.
Check the Devices using your Google Account
If you want to check what devices (computers and smartphones) are using your Google Account, the steps are similar. Just open up this security activity page while signed into your Google Account to be get a list of all the devices using your account.
As you can see in my own, Google identifies two Windows machines and a single Nexus 5 on my account. The Nexus 5 is the smartphone I use actively, hence its usage just one hour before the taking of that screen shot. Check this list for devices that you don’t think should be using your account. Pay close note to the number of devices, the locations of those devices, and when they were last used. If you notice anything suspicious, you should update your password and secuity settings from “here”. Be safe.