You can achieve what you want without the instanceof keyword as you can write custom type guards now:

interface A {
    member: string;
}

function instanceOfA(object: any): object is A {
    return 'member' in object;
}

var a: any = {member: "foobar"};

if (instanceOfA(a)) {
    alert(a.member);
}

Lots of Members

If you need to check a lot of members to determine whether an object matches your type, you could instead add a discriminator. The below is the most basic example, and requires you to manage your own discriminators... you'd need to get deeper into the patterns to ensure you avoid duplicate discriminators.

interface A {
    discriminator: 'I-AM-A';
    member: string;
}

function instanceOfA(object: any): object is A {
    return object.discriminator === 'I-AM-A';
}

var a: any = {discriminator: 'I-AM-A', member: "foobar"};

if (instanceOfA(a)) {
    alert(a.member);
}