Assuming your opponent is waiting on 58 wan, his hand must have 67 wan each.
If you can see(including your hand, discards and called tiles) four 7 wan, then there's no way his hand is holding 67 wan. 8 wan can only be a shanpon or tanki wait, making it safer.
This 7 wan is known in Japanese mahjong as [kabe], 8 wan is known as no chance tile.
The degree of safety of a no chance tile is equivalent to that of a word tile, therefore the more 8 wan you can see, the safer it is. (If three can be seen, the last one is 100% safe.)
Also, it is known as [usu kabe] if three 7 wan is seen, 8 wan is known as one chance tile. The risk of one chance is between a suji-pai and non suji-pai. The important thing to note is, as the game continues, the degree of safety for one chance gets lower and lower, as the chances of drawing the last 7 wan gets higher and higher.
Fusion of kabe and suji
Effective use of kabe and suji can help determine more safe tiles, listed below is a few common examples:
a) If 4 and 7 are both kabe, then 5 and 6 are the same as word tiles, they can only be tanki or shanpon waits.
b) If 9 has been discarded, 6 is half suji-pai. If 4 is a kabe, because 45 cannot form with 6, the degree of safety for 6 is equivalent to double suji-pai. It would be even better if 5 is a kabe, as 6 is on the same level as suji of 1 and 9.
Kabe and suji have roughly been discussed. The next time will be introducing tile risk table.
(To be continued)