How do you distinguish between a suspension and a dominant 7th chord in the listening exam?

A suspension more often than not, will in fact contain within it a dominant 7th, so getting to know the difference between the two can be difficult! The key difference is that a suspension is contained of three parts; 

1. The preparation 

2. The suspension itself 

3. The resolution

The note that is causing the clash (the note which is not in the chord; the suspension itself) has to be harmonised within the chord that proceeds it. For example, if you were to have F major ( F, A, C) then G major7 (G, B, D, F) which resolves to C major (C, E, G), this would be a suspension. The F is the cause of the clash in the second chord, it has been harmonised in the chord previous (F major) and then resolves in the next chord to a G which is the fifth in the C major chord. What is also significant is that the resolution occurs by falling step, this is also a common feature of a suspension. 

By contrast, a dominant 7th chord where the clashing note is not harmonised in the bar before and which doesnt resolve by step in the following chord, is not a suspension but just chromatic colouring or an interesting harmonic feature.