Most observant jews that travel on business will eat fish at restaurants, often without knowing how to do it right and thus with a side order of guilt.
Here are the issues.
Bishul Akum for one. (Cooking of a non-Jew). Any of the fishes that are eaten raw in sushi, would not be a problem. If a Jew works there or owns the place, you’re good, even if not religious otherwise, ask to turn on the flame
Which fishes are kosher and which are not. Use google or check if they have fins and scales.
The pot or pan or grill they use will be clean. Only eat at clean places. If you’re not sure, check.
Ask what other ingredients are used. There are too many allergies these days followed by lawsuits. No one will lie to you.
enjoy.
yaacov wolf 
Vicki Shammah