class1.js

/* @flow */

export class GraphQLSchema {
  getQueryType() {
    return "foobar";
  }
}

class2.js

/* @flow */

export class ThirdPartyGraphQLSchema {
  getQueryType() {
    return "fizfuz";
  }
}

types.js

/* @flow */

export interface IGraphQLSchema {
  getQueryType(): string
};

usage.js

/* @flow */

import type { IGraphQLSchema } from './types';
import { GraphQLSchema } from './class1';
import { ThirdPartyGraphQLSchema } from './class2';

export function printSchema(schema: IGraphQLSchema) {
  console.log(schema.getQueryType());
}

printSchema(new GraphQLSchema());
printSchema(new ThirdPartyGraphQLSchema());

validators.js

/* @flow */

import type { IGraphQLSchema } from './types';

export function isGraphqlSchema(schema: IGraphQLSchema) {
  return typeof schema.getQueryType === 'function';
}