It depends on the protocol you use for the vpn. The more "standard" ones use PPTP and IPSEC. These use protocols that are pretty much only used for vpn so it is obvious. Most VPN services still support those but many now use what is called OPENVPN. This uses the same ports as HTTPS and since HTTPS is extremely common it looks like most other traffic so they can not easily detect it.
Now countries like china have figured out that OPENVPN does not follow the HTTPS standard directly so they can detect it by watching the messages sent when the tunnel is established. They have even found patterns that they think are unique to SSLVPN which actually does follow the HTTPS standard and is used by corporate gateways.
OPENVPN does not...