Tor: Can my internet provider tell that I use Tor?

Short answer

With a normal direct connection, a provider or local network observer can usually recognize that you are connecting to Tor, but cannot see the websites you visit through it. A bridge and pluggable transport can make Tor use harder to identify and can help bypass blocking. They do not hide all traffic facts or the possibility that your device is using a circumvention tool.

Sources

Tor Project: Tor Browser best practices, Tor Project: Using bridges.