Recent versions of Chrome will show a warning if you try to browse to a site that uses a SHA-1 certificate. Mozilla is doing the same thing as of Firefox 51, but they're enabling this gradually to measure impact. Microsoft has an update ready to disable SHA-1 support in their browsers - I think it was delayed a few days ago due to some issues (not sure if they were related to disabling SHA-1).

You can use [1] to test how your browser behaves.

Technically, the sites cannot be said to be vulnerable because of their SHA-1 usage. Rather, continuing issuance of SHA-1 certificates by publicly-trusted CAs increases the risk that someone obtains a certificate that collides with a certificate for a different domain or for a certificate that could be used to sign other certificates for sites the attacker does not own. [2] does a good job of explaining this. The mitigation for this is to use a browser that disables (or warns about) SHA-1 certificates. Publicly-trusted CAs are also not supposed to continue issuing these certificates, but there have been quite a number of cases where they did so anyway - most notably WoSign.

Of course, a site might use SHA-1 for other things behind the scenes. There's really no way to detect that in general.

[1]: https://sha1-2017.badssl.com/

[2]: https://news.ycombinator.com/item?id=13715717