I assume this is derived from other results of the campaign. I see two methods:
1. Retrieve an Adhoc report with the fields SearchBudgetLostImpressionShare and ContentBudgetLostImpressionShare. When either field indicates that indeed impressions were lost, the budget is not high enough.
2. Retrieve an Adhoc report with the fields Cost and Budget. Compare them, when Cost > 95% of budget then you can assume that the budget is limiting the number of impressions.
For both methods you need to supply a period, and you need to experiment with this. (eg. last 7 days, this month etc)
Note: The second method is tough to implement when you use shared budgets.
Hope this helps