There are times that you cannot access a WordPress website at all. White Screen, Fatal Errors, you name it.
You can access via FTP and disable plugins by renaming the plugins folder. The same with the themes.
However, from outside of the WordPress Dashboard you cannot really tell what is the active theme.
If you have hosting access, go to the databases, and enter phpMyAdmin. Locate the site’s database.
Go to the wp_options table.
Navigate through the registries until you find a row with the option name set as ‘template’.
In the option_value column, you’ll see the name of the theme in question.