Recently, I published a series of posts describing the various strategies students can use for comparing fractions. While creating a common denominator is one of the strategies, it is often not necessary. For example, consider this pair of fractions:
I have been working with my 4th graders on this skill, and I created an anchor chart for them to use as a reference when comparing fractions. Please, PLEASE remember that students need lots of concrete and pictorial experiences with fractions to be able to reason about the relative size of fractions, which is why I included visuals on the anchor chart. You can download a copy of the anchor chart here.