When using fraction strips how do you know that to fractions are equivalent

You basically would use how long they are. If you had a strip of 1/2 and a strip of 3/4... with blocks they wouldn't be the same length. The 3/4 blocks would be greater.

However if you matched a 1/2 strip with a 3/6 strip, you can see that they are the same length and are equivalent. 

Of course their is a way to do it mathematically but let me know if you want to see that.