How to know if I am about to hit the org governor salesforce limit class?

The way I see it, you have two options to avoid hitting or know when you are about to hit the org governor salesforce limit class.

Display your message whenever you end up with 0 remaining query rows available. Write a batch that will store the number of records that object contains in a Hierarchy Custom Setting.

Either way, it will probably simplify repeated references to add a helper method:
public static Integer getRemainingQueryRows() { return Limits.getLimitQueryRows() - Limits.getQueryRows(); }
For Option 1 your solution would simply look like:
someList = [ SELECT convertCurrency(Price__c) FROM MyObject__c LIMIT :getRemainingQueryRows() ]; isQueryLimitConsumed = (0 == getRemainingQueryRows());
For Option 2 your current implementation would change to look more like:
isQueryComplete = MySetting__c.MyObjectCount__c > getRemainingQueryRows(); someList = [/*query*/];