The answer to this used to be fairly simple: the old fw ver command. It's still useful, of course, but the answer today is a little more complicated.
From clish in Gaia, a show version all will show you the OS build (take), kernel, and whether 32-bit or 64-bit OS is installed.
If you've loaded a recent Jumbo Hotfix on R80 or earlier releases, the command installed_jumbo_take will give you the "Take" number of that Jumbo Hotfix.
In R80.10, the command cpinfo -y all will show you what jumbo hotfix is installed as well as any other (one-off) hotfixes that may be installed. Note this command works on earlier releases as well.
Reference: Jumbo Hotfix Accumulator FAQ