I'm assuming that since you sent your tablet for repair that it was pretty much unusable, so the most likely case is that the repair people decided it was a lost cause, chucked it in the recycle pile, and sent you a new one. It's unlikely that any attempt was made to access your accounts. I would not proceed on that assumption however. As for how to see if anyone has tried to access your account there's really no way to be sure.
The fact is that you can still access your account, so no-one has changed your password on you, and you still have control over your account. So the first thing to do is change your password, right away. Anyone who may be using the cached credentials in your device will no longer be able to do so. If someone had tried to take over your account they would have changed your password and the secondary email account to keep you from re-setting the password.
Before I stop using a mobile device I always wipe it, plus any additional storage on the device, however when a device breaks you may not have the ability to wipe anything. In that case all you can really do is change all your passwords before sending the device in, and hope that nothing personal can be recovered on it.