In my opinion the question should not be "How to know ...if"; it is more suitable asking "When should I setup Jumbo frames".
As per your answers about MTU/Jumbo frames:
1) To see your MTU, either
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.1.249 netmask 255.255.255.0 broadcast 192.168.1.255
ether 00:0c:29:40:68:ee txqueuelen 1000 (Ethernet)
RX packets 50182 bytes 22054712 (21.0 MiB)
RX errors 0 dropped 3 overruns 0 frame 0
TX packets 6674 bytes 838613 (818.9 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
$ ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 00:0c:29:40:68:ee brd ff:ff:ff:ff:ff:ff
inet 192.168.1.249/24 brd 192.168.1.255 scope global eth0
valid_lft forever preferred_lft forever
Or more simply:
$ ip link show eth0 | grep mtu
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
2) Normally in IPv4, the default is MTU being 1500 (bytes).
3 and 4)
Usually the default MTU is fine. Rarely you have to change it for lower, temporarily, when dealing with communications problems, when firewalls are blocking ICMP packets which do not allow MTU to be negotiated.
The other situation you may wish to change MTU is for defining JUMBO frames.
However, you do not benefit alway of using JUMBO frames, only in specific cases like a dedicated VLAN of file servers or maybe DB servers, and on top of that, in a VLAN with JUMBO frames ALL the machines have to be configured for JUMBO frames; the switches also have to support JUMBO frames.
You surely do not want to use JUMBO frames for Internet facing systems, like web servers or DNS servers.
For setting in run time an interface for JUMBO frames:
ip link set eth0 mtu 9000
/etc/network/interfaces (in Debian) for making it permanent:
iface eth0 inet static
For RH based systems: