How can I check the last time someone logged into my Gmail account?

There is indeed a neat way that Google makes it possible for you to see a history of who logged into your account, from where, but before I show you that, let me say that the #1 rule for anyone who thinks something funny is going on with your account is:

Change your password.

Now.

Okay, you're back? Good. Changing your password is a very good - though not infallible - way to ensure that everyone else is locked out of your account and should fix the vast majority of sneak attacks.

Now, log in to your Google Gmail account and scroll down to the very bottom of the main window. There you'll see:

As you can see, the last time my own Gmail account was logged into was from the same IP address I used this time, which is as it should be. More interestingly, click on Details and you'll see a history of logins:

I access my Gmail account both through my mobile and main computer, so this makes sense and is accurate. If someone else were also logging in to my account, I'd be able to see it here on this screen and then could map backwards with IP address (Google "reverse IP lookup" to find some tools to help with that).

If you were logged in to more than one location at once, by the way, here's what you'd see:

Armed with that, you should be able to get a better sense of whether someone really is logging in to your Gmail account or not. Good luck!