One (old) way is to use the colour depth and/or resolution combinations to determine the total VRAM.
NewerTechnology's Guru allows you to select a resolution (under the 'video' tab) which will then show the colors available depending on the amount of VRAM installed. Compare that to the available settings in your Monitors control panel, and you should be able to determine how much is fitted.
Note that Guru does NOT query your system and it's purely for discovering possible configs.
https://www.newertech.com/downloads/Software/GURU29Installer.hqx
There are only two configurations for the LC475 -
2x 256K SIMMS for 512K VRAM
or
2x 512K SIMMS for 1MB VRAM
So, check your monitors control panel for the following indications:
With 512K, 640x480 resolution can do '256' colors (8bit color)
With 1MB, 640x480 resolution can do 'Thousands' of colors (16bit color).
Be aware that mixing a 256K and a 512K will result in ONLY 512K VRAM, so matching the SIMM designs and chip numbers is fairly important.