#JavaScript (ES6), 104 102 bytes

Returns a Promise containing a JSON object for truthy or undefined for falsey.

n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>j.items[0])

##Try it

f=
n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>j.items[0])
oninput=_=>f(+i.value).then(console.log);f(i.value=1).then(console.log)

<input id=i type=number>

JavaScript (ES6), 104 102 bytes

Returns a Promise containing a JSON object for truthy or undefined for falsey.

n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>j.items[0])

Try it

f=
n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>j.items[0])
oninput=_=>f(+i.value).then(console.log);f(i.value=1).then(console.log)

<input id=i type=number>

#JavaScript (ES6), 104 bytes

n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>!!j.items[0])

##Try it

f=
n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>!!j.items[0])
oninput=_=>f(+i.value).then(console.log);f(i.value=1).then(console.log)

<input id=i type=number>

#JavaScript (ES6), 104 102 bytes

Returns a Promise containing a JSON object for truthy or undefined for falsey.

n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>j.items[0])

##Try it

f=
n=>fetch(`//api.stackexchange.com/questions/${n}?site=codegolf`).then(r=>r.json()).then(j=>j.items[0])
oninput=_=>f(+i.value).then(console.log);f(i.value=1).then(console.log)

<input id=i type=number>