Is there something you can look for? Am I left to the seller's word and my own discernment?
General food safety guidelines give you four hours of time between 40F and 140F; more than that, and unsavory bacteria exceed nominally safe limits.
So, there's your safety limit. Total time counts, that is, in a bag back from the grocer for 2 hours, then in the fridge and back down to 34F and then out for 1 hour on the counter = 1 hour left until it needs to go over 140 for ten minutes (or 165 for 1 minute) or into your mouth.
And of course, there's the amount of time it was left on the dock, etc. etc.
That said, if you left fish in the fridge for a week, it would be mushy and bad tasting raw -- so you also must use your tastebuds and nose to see if you want to eat it raw; food safety isn't the only concern.
Well, your discernment is indeed what you are looking for. You want really fresh fish. The usual advice for detecting really fresh fish apply. It can't hurt to have a fish source that you have good reason to trust.