I work underneath a certain airway near the local airport, and airplanes fly almost directly over my workplace, often low enough to make out some details. Naturally, I can only see their bellies from below. Unfortunately, the landing gears are stowed at this distance (otherwise it would be too easy). While there are a ton of excellent resources on this site and many others, they don't necessarily focus on spotting planes from below.
Understandably there is a similar question asked on this site. However, (1) the question implicitly refers to silhouettes of planes flying high(er) up in the air, and (2) the answers all refer to external resources (i.e. books).
I would like to see some pictures illustrating the differences of bellies among common air transport aircraft models at whatever altitude that is typical around 5-10NM from the runway. (That would be 1600-3200ft assuming 3 degree glide slope.)
So far I can only make out engine chevrons on Boeing planes, count the number of engines and flap fairings (is there any pattern to the number of fairings?), look up the registration number painted under the wing (duh), and not much else.