Checking model scale while shopping is difficult without a pocket sized measurement check for different scales, especially if you are looking for items in unusual places. One of the easiest things to use as a scale check is a paper person. They are lightweight, easy to pack, and we can all relate to their size. Looking for chairs in particular scales? They should have a seat that falls just below your knee. How big would that tree be in your scene? How many man heights is it?
I carried around a plastic coated figure from a catalogue until it finally wore out. Now I use a known item, my doll house garden boy George. George is a bit hard to pack around on a daily basis, so I set him up as a photo reference in two sizes across a range of common miniature scales, for everything from dolls house items to gaming and railroad figures. In each scale, the printable version of my garden boy is in scale at six feet high, and one at five foot six.
Scales for miniatures and models are not always set to the same ratio. Some scales, like 'N' and 'G' railroad scales can vary across manufacturers, or are different from country to country, if scale is important for a particular scene, make sure you know exactly what ratio your collection of miniatures is.
Scales for miniatures are relatively recent. Until around 1960, many doll houses used scales of 1:16 and some newer 'toy' houses are being reproduced in this scale. Many plastic models are sold in 'fits the box' scales. Scales may not be as important as the size of the packaging.
Small adjustments in scales don't throw a scene out too much, especially if you can use the size difference to trick the eye into thinking it is seeing a longer perspective by putting larger objects closer to the viewer and items which are smaller than the size they should be towards the back of a scene.
Consider the time period when checking scales. Historically, the height of people has varied (and not always been on the increase surprisingly!) In 1970, the average male height in the U.S. was five foot ten inches. In the late 1700's it was five foot eight inches. In between those dates it dropped a surprising amount.