Which command is used to verify VPN connection is operational (or something like that) ?

Answer: crypto ipsec sa

Explanation:
BD
#show crypto ipsec sa – This command shows IPsec SAs built between peers
In the output you see
#pkts encaps: 345, #pkts encrypt: 345, #pkts digest 0
#pkts decaps: 366, #pkts decrypt: 366, #pkts verify 0
which means packets are encrypted and decrypted by the IPsec peer.
Source: http://www.cisco.com/c/en/us/support/docs/security-vpn/ipsec-negotiation-ike-protocols/5409-ipsecdebug-00.html#ipsec_sa