You can use instanceof operator:
var t = /^foo(bar)?$/i;
alert(t instanceof RegExp);//returns true
In fact, that is almost the same as:
var t = /^foo(bar)?$/i;
alert(t.constructor == RegExp);//returns true
Keep in mind that as RegExp is not a primitive data type, it is not possible to use typeof operator which could be the best option for this question.
But you can use this trick above or others like duck type checking, for example, checking if such object has any vital methods or properties, or by its internal class value (by using {}.toString.call(instaceOfMyObject)).