Best way to check conditional interfaces

export interface YouI {
  from: 'You';
}
export interface MeowGPTI {
  from: 'MeowGPT';
  isPrinted: boolean;
}
export type ConvI = {
  message: string;
  id: string;
} & (YouI | MeowGPTI);

What is the best way of checking the inner conditional type instead of doing this

if("isPrinted" in obj)