I apply the same metric I apply to online product or service reviews. If they appear on a regular cycle, or if they appear in some of there kind of statistically improbable clumps, or if there are no negative comments at all, then they may not be entirely plausible.

Employment verification won't help on Glassdoor, because it would be the easiest thing in the world to hint to a group of the most settled employees that maybe they could leave positive ratings.

It would help against malicious negative reviews - which can also be a thing.

Many valid reviews are written by former employees, so I'm not sure how you could verify those. A subset of those are from people with an axe to grind. Look at any company that's had layoffs, for example. In general, I've found Glassdoor to be incredibly biased, either shills (in a small company, nobody's going to publicly trash their employer with the risk of being identified), or trolls (any company, once large enough, has former employees who didn't get along with their boss and choose to escalate it).

Relatedly, I'm not convinced Glassdoor doesn't delete or hide posts that are unfavorable to the employer. I've kept tabs on the reviews and scores left on my previous company's page and at least several negative reviews are nowhere to be found.

Fake reviews are generally written by PR/Marketing people so they never look authentic.

A lot of companies ask their employees to write positive reviews so employment verification doesn't necessarily help guaranteeing authentic reviews.

I think it requires a company email? I can't recall. They also have a verification window, reviews don't post for a couple days.