How to check one user's permissions for the root site and all the subsites?

we are using O365/SharePoint Online 2013.

Is there any way to check a specific user's permissions for the root site in question and all of its subsites so basically all sites in a site collection? We have 10 subsites and some users can access a few of them and some might have the access to every one of them.

From time to time I get asked to check where one specific user has access to and at the moment the only way I have found is to check the user permissions one site at a time. That is quite time-consuming, so is there any easy/quick way to see all the sites a user has access to?

I greatly appreciate any information regarding this matter!