Use the [If-Modified-Since][1] header to check for changes. Basically, when performing a describe by REST, this header lets developers know if there's been any changes since the last describe.

> Use the SObject Describe resource and the If-Modified-Since HTTP header to determine if object metadata has changed.

> You can include the If-Modified-Sinceheader with a date in EEE, dd MMM yyyy HH:mm:ss z format when you use the SObject Describe resource. If you do, response metadata will only be returned if the object metadata has changed since the provided date. If the metadata has not been modified since the provided date, a 304 Not Modified status code is returned, with no response body.

 [1]: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/sobject_describe_with_ifmodified_header.htm