How is it possible to check data usage and see whether the limit is approaching?
The limits and their threshold values are documented here.
In most cases it's fairly obvious which limit has been breached. In rare cases when it's not obvious you can generate SxS log as described in this article. When the trace level is set to the maximum the log file will contain details about every request submitted using Eikon Data APIs. For every request that was denied the log file will tell you the reason, i.e. which specific usage limit was breached.