The fact is that with startups there is no way of knowing what is the truth. Particularly, the runway that they have is something they wouldn't share with a prospect.
Best is to reach out to employees who had left the organisation recently. Track the lowest rated reviews on Glassdoor and look for employees who had previously left this organisation. This helps more often than not.
Present employees lie as they feel obliged to their present employer. This is also because they feel that there is a face chance that the person who is asking for reviews will join them shortly, creating complications for them in the workplace later.