As the expansion rate of coolant is well understood, the free space required to accomodate the expanded coolant is therefore very easily calculated. Moreover, once the ideal cold level is understood and set, it only takes a quick nosey in the tank to establish all is well. If you think about it, it's really no different to the regular checks we all make on the level of oil in our sump, a sump is no more transparent than my aluminium expansion tank, and sump also doesn't have a sight glass.
Any catastrophic loss of coolant would be acompanied by a very distinctive smell, and you'd be pulling over pretty sharpish if you caught a whiff of that. In my experience, Monty has never once called for a drop of additional coolant, in the five years I've had him his coolant level has always remained extremely consistant, which TBH is no different to every other car I've ever owned with a sealed system.
I'm not suggesting I won't check the coolant level, but failing any telltale smells, I'm confident a quck peak under the cap once a month would be more than sufficient. I check Monty's sump level on the dip stick more regularly than that, so popping the cap off my aluminium expantion tank and having a quick look inside should hardly be considered an inconvenience.
The way I see it, it's just another occational pre-flight check that brings man closer to his analogue machine. In an increasingly digital world, I view these types of checks as all part of the theraputic pleasures associated with owning a classic car
