I would recommend not only check timing... Timing is important, but also is important to develop a "game feeling"!
For example, if you play vs Zerg and 'usually' are expecting mutas around 9 minutes... you probably will build 1-2 cannons/turrets/spore crawlers (that is ok not only for low level players) in each of your mineral line (you should have main + natural till that time).
BUT, if in the early game you did good damage to your opponent he won't be able to get those mutas as usually. They will be 3-4 minutes later. So instead of investing 4-6 hundred minerals in those static defense right now ... you would better expand or build 1-2 ore production buildings. But don't forget about that defense 2 minutes later.
Another example: your opponent builds a bunch of infestors. For this case, I would say: don't care about mutas at all! If you see at least 1-2 infestors (or infestor pit), that usually means he will have 3-5 infestors more. In turn, that means either no mutas at all, or just a very small amount of them (at least in the nearest time) and you can deal easily with them with no static defense.
The best way to know timing is to have game feeling. To do that - you need to see an opponent army constantly. To do that in turn - harass constantly: don't waste your army for nothing, but keep opponent under pressure. So his mutas will go not to mineral line, but to your death ball...