If you are retaining excessive amounts of fluid, you will most likely be able to see it. Your ankles may be swollen and puffy by the end of the day, watches or rings may feel tighter and leave a ring, your face can appear puffier.
Generally, drinking a good amount of water will help you to retain less fluid, and it's more often an issue with women more so than men.
If you aren't showing any signs of excessive fluid retention, I wouldn't worry about it, just weigh yourself under as similar circumstances as possible to minimize the variables.