Can I check if a field name is number in N1QL?

N1QL
1

Or generally can i clarify if a field name has a particular type? For example
“jenny” : “nig” ,
"300" : { . . .
}
I would like to check if “300” is number.

2

Field names always string. If number is stored as string you can access with back-ticks

SELECT d.`300` 
FROM  default AS d 
WHERE .......

IS_NUMBER(TO_NUMBER(“1234a”)) returns true if given string is number.
Type() give type of value.

You can use following expression

OBJECT  v: CASE IS_NUMBER(TO_NUMBER(v))  THEN "number" ELSE "string" END 
FOR v IN OBJECT_NAMES(d) 
END