Veneer can be mistaken for solid wood, especially if the grain is good and the finish is partially present. The type of material matters because it is just a thin layer of wood above other materials. The layer is thin so it is easy to damage. A table top, cabinet door, drawer front, etc., may have only a small area of actual surface to sand before revealing the bottom of that layer.
Usually, there are areas where the top layer is visible from the side. Look at corners, chips, drawer fronts, and the edge of the underside of the table. A thin line that is different from the color or material below it may indicate veneer. In some cases, the veneer is peeled, bubbling, cracked or missing on a corner. Sometimes, you are looking at an otherwise smooth veneer that is already thinly sanded due to repairs. Use a light side light while examining the piece, or you may miss spots that can be damaged before starting sanding.
A first safe thing you can do with many veneered pieces is cleaning. Old wax, oil polish, grease, and dirt can make veneer look dull. Use a soft cloth and a gentle cleaning method on an inconspicuous spot and allow it to dry completely. If the grain is clearer on clean dry surface, your item will probably require less sanding. This is because it is possible to sand through a thin layer of veneer and leave a lighter colored spot, a darker spot of the underlying material or an uneven spot, which will show up under stain or a clear coating.
Control your pressure with veneer, rather than speed. Only use the sanding block on a completely flat surface, and keep your sandpaper moving in the direction of the grain, do not force any scratch to disappear. Deep scratches, water marks or stains are usually below the surface that can be sanded. In an attempt to erase any mark entirely, you may remove some of the actual layer. It is much more desirable for damage to look like damage instead of a complete repair, as this gives the whole piece a cared-for look.
The issue with a loose edge is different from a damaged finish. When the edge is loose, do not try to apply any filler in the gap; it will not help the loose surface. The layer may need gluing and clamping, with some cracks being too fragile for a first-time job. A corner that is small and peeled on a practice piece will show the delicacy of the veneer surface to you, but a large area on the top that is peeled, a missing area with a patterned veneer or antique veneer is a surface to think carefully on before doing a quick fix.
Finish testing with veneer can help. Stains, oils, waxes, paints, washes or clear coating can change how thin wood looks. Any unevenness in sanding that may seem fine will be darker under stain or other finishes. Test in an inconspicuous area if one is available. If there is no inconspicuous area on the item, test a different surface to test brush or rag application, then do your best on the piece in as light of a touch as feasible.
Ask the question: Am I trying to improve the surface, or do I want to remove any damage? For a beginner’s project, improvement is a better goal. Clean it, find out if there is any other type of surface to sand, sand lightly, sand with grain and stop before things begin to get worse. Veneer works with care more than strength.