If you go to Stack Management > Index Patterns, you should be able to get an ID of your index pattern from the URL. URL will be something like: <your_hostname>/app/management/kibana/indexPatterns/patterns/ff959d40-b880-11e8-a6d9-e546fe2bba5f
The long alphanumeric value after patterns is your index pattern id; in this case index pattern ID is: ff959d40-b880-11e8-a6d9-e546fe2bba5f
Then, you go to Saved Objects, also under Stack Management. Find the visualization you need, and then from a pop-up selector on the right choose Inspect. The last section in there should be references, it will show current index pattern id:
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