方法一：  网络适配器设备状态“已连接”的对勾已经勾上，说明网线已连接

 方法二： 虚拟机右下角，右键单击网络适配器，显示“断开连接”，说明网线已连接

方法三：输入命令mii-tool 网卡名，显示link ok，或输入命令ethtool 网卡名，显示Link detected: yes，说明网线已连接

[root@centos7 ~]#mii-tool eth0
eth0: negotiated 1000baseT-FD flow-control, link ok
[root@centos7 ~]#ethtool eth0
Settings for eth0:
	Supported ports: [ TP ]
	Supported link modes:   10baseT/Half 10baseT/Full
	                        100baseT/Half 100baseT/Full
	                        1000baseT/Full
	Supported pause frame use: No
	Supports auto-negotiation: Yes
	Supported FEC modes: Not reported
	Advertised link modes:  10baseT/Half 10baseT/Full
	                        100baseT/Half 100baseT/Full
	                        1000baseT/Full
	Advertised pause frame use: No
	Advertised auto-negotiation: Yes
	Advertised FEC modes: Not reported
	Speed: 1000Mb/s
	Duplex: Full
	Port: Twisted Pair
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	MDI-X: off (auto)
	Supports Wake-on: d
	Wake-on: d
	Current message level: 0x00000007 (7)
			       drv probe link
	Link detected: yes
[root@centos7 ~]#