Well, their REST API isn’t very standard. The standard way to do this is oauth2. When you are using oauth2 when your auth token expires you will get a 401 return code.
Searching around that doc it appears that they are at least doing that much. So the approach would be to make the call, if it returns a 401 instead of 200 return code you know the token is invalid and you need to go get a new one. You can see a similar example using oauth2 at OAuth2 using just OH Rules and myopenhab.org.