To know your eye power also known as refractive error you will have to see an eye doctor by scheduling an eye exam.
The eye doctor or optometrist or ophthalmologist will perform some eye tests such as retinoscopy and visual acuity, which measures how well the eyes focus light and determine any necessary lens correction.
The eye doctor will perform a visual acuity test which is often done using the Snellen chart that measures your ability to see letters or symbols at a specific distance which helps to determine if you have 20/20 vision or require corrective lenses such as eyeglasses or contact lenses.
A refraction test will also be done by the eye doctor which uses a phoria or autorefractor that helps to determine the power of lenses that are needed to correct any refractive errors such as myopia also known as nearsightedness and hyperopia also known as farsightedness or astigmatism.
Another test that is done by your eye doctor for determining eye power is a retinoscopy which is a test that uses a handheld device called a retinoscope to observe your retina under a beam of light which helps to determine your eyes refractive state and the power of any lenses that may be needed to correct the vision.
An autorefractor eye test uses and automated device or autorefractor to measure the refractive error of the eye by analyzing how the light reflects off your retina and helps to determine the power and type of contacts or prescription glasses if needed.
And a visual field test is a test that measures the range of your vision and helps to identify any loss of vision or other eye conditions.
Also the numbers that you see that are written in your eye prescription under abbreviations such as SPH, OS and OD determine your eye power.
The numbers range from (+/-) 0 to 20.