You can check the gravity of your wine using a hydrometer.
Gravity is an indication of how much sugar is suspended in a solution, and as yeast consumes the sugars in your wine to produce alcohol and CO2, the gravity reading goes down.
After 5 days of fermentation, take a sample of your wine and to check the gravity. If the reading is equal to or less than the number stated in your instructions, fermentation is complete. To be absolutely sure, you can take another reading 24 hours later to ensure the gravity is stable.
If you don’t have a hydrometer, you can taste the wine to check it’s completely dry, however, this isn’t as accurate as using a hydrometer.