At first, the direction of cutting edge.
Most of tsuba should be put the cutting edge upward to appreciate it. They are tsuba for Katana, for Wakizashi, and for Tanto.
The obverse is, Kogai hole right side and Kozuka hole left side. The other face is reverse.
(Usually, Kozuka hole is a simple arc or oval, Kogai hole is shaped with three arcs.)
Most of Wakizashi tsuba and Tanto tsuba have only Kozuka hole, in such case, put Kozuka hole left side.
The obverse is a front face when samurai puts it on his waist.
 
Katana tsuba
  

Wakizashi tsuba
  
 
Tanto tsuba

 

- Studying SEPPA-DAI -
In the case of symmetrical designed tsuba, please study the SEPPA-DAI.
Seppa-dai means a base of seppa. On the obverse, it is a base for Fuchi. On the reverse, it is a base for habaki.
Therefore, seppa-dai in reverse should be flat. When the tang hole is punched to make it small, it has to be done from the obverse side to keep the reverse side flat.
Seppa-dai in obverse can be hollowed by punching. Sometimes, it is originally hollowed to have a good fit with fuchi.
 
Obverse of tsuba(s) what have punched seppa-dai
    
 
Exceptional tsuba(s)
This tsuba is symmetry and no kozuka/kogai holes. It can be used for either Katana or Tachi.

 
This tsuba has only kozuka hole. It is not a kogai hole. The shape of kozuka/kogai holes are not an exact rule.

 
This is a simple square tsuba of custom-made. Around the tang hole is slightly hollowed on the obverse.

 
 
-Signature of smith-
Some tsuba smiths put their signature on seppa-dai. But there is no rule to put a signature on obverse or reverse. Some smiths put on the obverse, and some put on the reverse.
In old days, before Edo period, many smiths put his signature on the obverse.
Later, in Edo period, some smiths put his signature on the reverse. Because they don't want their signature damaged by punching.
 
A signature in the obverse seppa-dai. "Efu ju Yasutsugu zo"

 
A signature in the reverse seppa-dai. "Shoami Shigetsugu saku"

 
 
TACHI TSUBA and NAGINATA TSUBA
They are cutting edge downward.
 
Tachi tsuba
This is a copy from a tachi tsuba in Nambokucho period (14th century).
If you put it cutting edge upward, the design becomes upside-down.

 
Naginata tsuba