Ideally, this would be from members of the target audience, although this may not always be possible. And the more people you can get feedback from the better.
You should also consider asking your test audience specific questions as this may help them to give more useful and specific feedback.
There are a number of ways you could gather feedback:
Once you have gathered feedback and improved your product you should always revisit your original success criteria. This allows you to consider how successful your product has been at meeting your objectives.
Go through each success criteria and evaluate how well it has been met.
You should also consider how you could improve the product if you were to make something similar again.
You may be able to use the feedback you got from other people to help with this.