The Gemara in Nazir entertains the possibilty of Moshiach having arrived and people aren’t yet aware. The Rambam writes that there are many and conflicting depictions of Moshiach’s arrival in Navi and Medrashim. We can’t tell until he comes, very soon.
The Mesora is that we will be notified beforehand. Also, since there is Moshiach ben Yosef and Moshiach ben David we will be aware by the time Moshiach ben David comes.