This is a simple thing to do - and I am surprised that the majority of businesses do not do this.
Support your radio ad with the same message in print from, from A5 flyers at your business, to posters and even newspaper ads (if you are targeting an audience who reads papers).
You may be able to also target more of your audience by using the radio ad in a podcast which is specific to your demographically targeted audience - you will need to speak to us about doing this is as there is a minor cost involved in releasing the ad for this use.
You can also target with an online ad buy, on google ads (or similar) and/or with Facebook ads.
By tracking this message - that is a direct reflection of your radio spot - you will be able to see how that message tracked with online users - just remember that not everyone is online - or on social media - so you are only able to track a small percentage of your audience.
But doing a media mix of radio and online has the potential to lift your sales by at least 23% (according to one set of research I have seen - but it is a few years ago now - and I think the numbers would have increased rather than decreased).
This mix will not only re-enforce your message, but also assist with branding in your customers minds.
Try it - it will increase your success rate by at least 23%!