There are times when you have to sharpen a saw blade if it has a large and expensive blade. Otherwise, it is better to throw it away and get a new one.
If I were you, I would first inspect the saw blade carefully for carbide teeth. Just to make sure none of them are missing, chipped, or broken.
There are some high-quality saws that come with really nice saw blades. I wouldn't attempt to resharpen them on my own. So, I would rather take it to my friendly neighborhood saw sharpening shop and have them look at it.
The guys at the shop will know better and be able to braze on new teeth. On the other hand, it probably isn't worth all the effort if the saw has an inexpensive big-box blade.
So, the question remains, how do you know if a hand saw can be sharpened or if it is intended to be disposable?
Check the blade first. If there are no damaged teeth, and the blade is just dull, then there is a chance that you can resharpen it. However, you should let your saw guy make the call.
He/she will be able to better if it is worth sharpening one more time or if it is time to get rid of it and get another one.
Take A Look At The Tooth Line
This is by far the easiest way to tell if your saw can be resharpened or you can dispose of it. Take a close look at the tooth line. Some saws have a dark strip right where the tooth line of the blade is. This means the hand saw has been induction hardened.
So, what does it mean? It means the teeth on the blade are just as hard as the file. There is no point in filing the existing teeth. For example, STANLEY Hand Saws come with induction hardened teeth.
If the saw blade gets dull, the only option left is to throw it away. It's also the easiest as well as the most economical thing to do. However, it is better not to buy a saw like this, to begin with.
When it comes to sharpening and filing hand saws, there are a variety of other factors that come into play. Don't bother resharpening a hand saw that has fine tooth blades for detail work.