How can you tell that the strength off both electromagnets is the same?

You can compare the force of attraction or repulsion exerted by each electromagnet on a common object, such as a paper clip. The electromagnet that can attract or repel the object with the same force is likely to have a similar strength. Additionally, you can measure the current flowing through each electromagnet using an ammeter to ensure they are drawing the same amount of electrical power and thus have similar strength.