Is it okay to eat meat if I don't know whether Bismillah was said during slaughter?

Yes — the default ruling for meat slaughtered by a Muslim (or Ahl al-Kitab, per Q8) is that it is presumed halal, and you are not required to investigate or confirm that Bismillah was specifically pronounced at the moment of slaughter before eating it. The majority view, particularly in the Hanafi school, holds that forgetting to say Bismillah does not invalidate the slaughter as long as it wasn't deliberately and intentionally omitted — the obligation is on the slaughterer, not on you as the consumer to verify it after the fact.

This connects to a broader principle in Islamic food law: the burden isn't on the eater to interrogate every detail of an animal's slaughter when buying meat from a Muslim butcher or a properly halal-certified source — reasonable trust in the source is sufficient, and excessive doubt (waswasa) over things you have no real way to verify is discouraged. If you have actual reason to believe the slaughter was deliberately done without invoking Allah's name (as opposed to simple uncertainty), that's a different and more serious situation worth raising with a scholar.

Source: general Hanafi fiqh position on tasmiyah (saying Bismillah) as obligatory but not invalidating if forgotten; general fiqh principle of default permissibility (al-asl al-ibahah).