unless it's a mask part it's not going to reveal them, even then some of them are especially difficult.
mask ROMs are basically 'set in stone' as it's part of the manufacturing process, it's more expensive, you typically have to make them in batches, once made they can't be changed, it's basically an array of fuses inside the chip. we can only see them optically *because* they're set in stone like this, the bits are physical.
for any part using regular erasable / programmable tech (or even on later 00s ones, flash tech) the bits are not visible, they're never going to be visible, they're electrical charges, not something you can see. all you do by decapping them is permanently render them unusable, with a die shot for future reference.
for smaller production runs it's cheaper (and lower risk) for a manufacturer to produce a huge batch of reprogrammable chips / dies then program them as needed than it is to produce batches of mask ROMs (which typically would have had a larger minimum order quantity) so once MCUs of that nature were an option, the majority used them as it made more financial sense than committing to a batch of mask ROM based MCUs.
even for non-glob MCUs found on arcade boards etc., the mask type is much less common.and again typically only found on earlier productions.
as I said, some of the most recent ones (from the last 5-10 years) even used flash tech, some of them are 100mhz+ ARM processors and 8meg of ROM under a tiny glob even if they're driving non-video, simply because that's what became the cheapest option (somebody I know worked on one about 10 years, and was amused by what a waste of tech and processing power it was)