First of all, you need to familiarize yourself with the spirit of the age in which the furniture was originally made. The furniture of previous centuries was handmade, and whereas the outward appearances of furnishings may have seemed perfect,  the backs of drawers as well as the insides were usually carelessly made i.e. crudely built, using tree sorts of inferior quality such as  pine or poplar wood.

Generally speaking, given that large spaces and lavish furnishings were symbols of prosperity rather than a reflection of practical needs, aristocratic mansions were huge.  In Northern Europe, due to the cold weather, the sizes of rooms and furniture tended to be smaller than in Southern Europe. Nonetheless, you should be suspicious of furniture that fits your rooms perfectly ‒ such furniture may not be genuine.

Some restoration works are very difficult to make, like the one in the photo below, but you can still undertake some furniture restorations without being an expert.    

Be aware, however, that whatever you do to a piece of furniture, it must be reversible, e.g. you should never use synthetic glue or anything that cannot be erased. Also, it is important to preserve the original paint. Usually, it will be enough to polish the furniture with cotton, using a specific product.