How to implicitly ask colleagues how long they have worked in this company?
Such questions don't need to be asked implicitly, just ask them directly. Of course, it seems that you have to ask around sometime.
It's really hard for you to talk, so pretend to ask him about the "entrepreneurial history" of his company. If he knows everything, you can compliment him and ask him when he came to the company. If he is not sure, he will say when he will come.