Tradicionalmente, a partir de um computador cliente, não é possível saber em qual vlan você está. Você pode verificar um endereço IP e uma máscara de sub-rede e determinar em qual sub-rede está o IP, mas não a vlan. No entanto, o wiki wireshark tem algumas informações interessantes sobre a captura de tags vlan.
When capturing on a VLAN, you won't necessarily see the VLAN tags in
packets.
For example, in at least some operating systems, you might have more
than one network interface device on which you can capture - a "raw
interface" corresponding to the physical network adapter, and a "VLAN
interface" the traffic on which has had the VLAN tags removed. The
OS's networking stack would be connected to the VLAN interface, and
that interface would appear to the networking stack to be an Ethernet
interface with a smaller MTU than normal (to leave room for the VLAN
tags). On those OSes, in order to see the raw Ethernet packets, rather
than "de-VLANized" packets, you would have to capture not on the
virtual interface for the VLAN, but on the interface corresponding to
the physical network device, if possible.
Here are some details on capturing VLAN tags on various operating
systems. If the OS or the network adapter driver won't allow the VLAN
tags to be captured, set up port mirroring (or "port spanning", as
Cisco calls it) on the VLAN switch and connect an independent system,
such as a laptop, to the mirror port, and don't configure the
interface attached to that port as a member of a VLAN.. You'll
definitely see the VLAN tags, regardless of what OS the independent
system is running or what type of network adapter you're using.
Linux
To enable VLAN tagging, you need two things: the vlan rpm (e.g.,
vlan-1.8-23) and the 8021q kernel module. Once installed, the vconfig
command can be used to create VLAN interfaces on an existing physical
device. For more info, see the vconfig(8) man page.
After your VLAN interfaces are set up and traffic is flowing, you can
run Wireshark and capture on the VLAN interface of your choice (e.g.,
eth0.100 for VLAN 100) or on the underlying physical interface (e.g.,
eth0). If you choose the former, you will only see frames destined for
that VLAN; if you choose the latter, you may see all frames or you may
see only untagged frames (if there are any). It depends on the NIC,
the NIC firmware, the driver, and the alignment of the moon and
planets. (A table enumerating the behaviors of various adapters,
firmware versions, and drivers might be useful. -Guy Harris)
If you are capturing on the host system where the VLANs are
configured, you will probably not see the VLAN tags in the captured
frames -- even if you capture on the physical device. The driver is
stripping the tags before the pcap library sees them. See the tech
note from Intel mentioned in the Windows section below. (Do Linux
drivers support getting VLAN tags, perhaps with a driver configuration
option or other option, in the same way that the Intel Windows driver
does? -Guy Harris) (e100 driver works great on 2.4.26 - Jaap Keuter)
Windows
Windows has no built-in support mechanisms for VLANs. There aren't
separate physical and VLAN interfaces you can capture from, unless a
specialized driver that adds such support is present.
So whether you see VLAN tags in Wireshark or not will depend on the
network adapter you have and on what its and its driver do with VLAN
tags.
Most "simple" network adapters (e.g. widely used Realtek RTL 8139) and
their drivers will simply pass VLAN tags to the upper layer to handle
these. In that case, Wireshark will see VLAN tags and can handle and
show them.
Some more sophisticated adapters will handle VLAN tags in the adapter
and/or the driver. This includes some Intel adapters and, as far as i
know, Broadcom gigabit chipsets (NetXtreme / 57XX based chips).
Moreover, it is likely that cards that have specialized drivers will
follow this path as well, to prevent interference from the "real"
driver.