Polkadot is a decentralized network and any project can decide to build on it. There is no central authority to approve or confirm this.

The websites polkaproject.com and dotspot.io list many projects that are building on Polkadot, but those are community websites, so we cannot guarantee their accuracy.

However, any project can claim to be building on Polkadot and the best way to decide the legitimacy of a project is to do your own research. This guide might help you in that regard.