When your project reaches the level in which the game already has all the main features you should think about tests.
If you can't afford to hire professional testers you might want to ask your friends, family or people related to the Gamedevu industry that are willing to help you. Personally, I think you should use both of these options.
When working with friends and family, you will usually have the opportunity to look at them during the game. This is a big bonus, because you can observe their reactions and draw conclusions even when they don't tell you anything valuable. There is also a downside to this situation. These people may not want to express their opinions, which may seem to them… Silly or not worth mentioning. You need to constantly encourage you to talk about everything that comes to mind about your game. Because a good obiór of the game largely depends on such small.
When you are able to enlist the testing of someone in the industry, you can count on some constructive remarks.
You have to reckon with the fact that you will hear many things that you don't want to hear about your work. All accept gladly, do not comment, do not translate, if you do not ask for it. Simply Zniechęcisz people who are trying to help you.
The worst thing you can do is test your game yourself. As a creator or co-creator of the game you are not objective for your work. You also have a lot of knowledge about the project, therefore you will not be able to assess the practicality of certain solutions used. You also have certain behaviors that may affect the operation of certain game features.
You can use examples of items that are not game errors, but may degrade the reception.
Some time ago I had the opportunity to test the game in which Shift was responsible for running. As the game promoted a quick move, I kept the non-stop keys for about two hours, causing pain.