It depends on the length of your transition. The longer time the transition takes, the more space it occupies in the timeline. You zoom in to the timeline to see the seconds and frames, or zoom out to see minutes and hours. A 3s transition will be small in a 2 hour video where you are zoomed out to see the whole video (in fact it should be invisible). IF you want an efficient way of noting the start and end of it, use a marker (simple marker), stretch the marker across the transition, name it for the transition and now you can zoom until you see the marker nice an big when you want to select the transition.
The default for transitions is 1 or 3s unless you create your own the old skool way.
Steps:
blade-edit mark the clippings. Leave clipping A where it is, move clipping b up to a new video track (unlink from audio if necessary), drag the edit marker of clip b to overlap onto clip A, apply your transition in for clip b, your transition out for clip A, and do the same for the end of clip b (if it ends before the end of the whole video).
Pre-built transitions take these steps out for you and give you something to grab onto so you can change it quickly.