how to define if I the surface of my cabinets are wood or veneer?
Look at the edge of the back - a veneer is often not finished at the (unexposed) rear - look for a joint about 1mm (or a bit less) on the edge.
Any place where there is a sharp edge may also have a defect in the joining.
Also, the bottom may tell you - is is pressboard/
OSB/plywood? Then it is a veneer - a solid board will show grain and only grain.
Also - look at price tag and/or age. Real hardwoods ceased being the norm after WWII.
A hundred years ago oak was considered the cheap stuff - those folks would die if they saw the crap in the stores now.
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General Questions Moderator
Can you see end grain anywhere - not always possible if all the joints are mitred - but if you can, then it’s almost certainly solid timber. There are such things as endgrain veneers, but they are used for decorative inlays and marquetry, not general cabinet making.
Also - on the edges - a piece of cheap board or particle board will need the front edge also veneered - you can see a fine seam along the sharp corner. A real board, you can see the same grain pattern wrap around, and the end if visible will show end grain.
If it’s older and used, some of the edges of veneer might be chipped or lifting. If it’s solid, look for chips that show the deeper wood grain. Also, old and used the planks in a solid piece might be starting to warp and come apart - look for seams in the flat pieces where the grain patterns meet.
I’m also thinking solid hardwood planking wider than 12 inches is very rare. check the wood grain pattern for width on expansive areas like tabletops and cabinet ends. Of course, veneer could be narrow.
If it’s solid wood, the grain on any two adjacent surfaces should “match”. With veneer likekly not.