THey were 40% silver, so the outside will look the same. Most the the time the edges you can see clad layers proving its not solid silver. Weighing it is the only way to prove it, but I have never heard of a real 1965 half being solid silver.
You can also flip them in the air or drop them on a hard surface (not too hard, lol) and hear the difference between a 40% and a 90%. Usually, the 40%ers look different on the edge, but I have some that look just like the 90% 64s. bob
Keep in mind the 1964 halves are not 100% silver either. They and the coins before 1964 were 90% silver. from 1965-1970 the halves were 40%.
The clad layers of 40% Kennedy Halves are 80% silver. So a 40% Kennedy Half may look like a 90% half if the 21%/79% (silver/copper) core is not visible.
Maximum weight tolerance on a 40% silver half is 11.9 grams and most of them will weigh noticably less than that. If it is 11.9 grams or more then a specific gravity test would be in order.
sure a 1965 90% silver half dollar could exist. there have been quite a few real 1965 90% silver dimes and quarters found.