Differentiating API sources (staging/production)

Hi there,

I've recently setup a series of campaigns, which I would like to test using staging users from our external service which create the leads initially. Would anyone have any advice on differentiating leads created by different sources? Unfortunately it doesn't appear possible to filter the campaigns smart list based on a specific API key (our staging and production servers use different marketo API keys). I see that I can filter by source type, but it isn't granular enough for this purpose.

My only thought is adding a new boolean lead attribute, something along the lines of stagingUser. This would require additional changes to our service naturally, but perhaps it is the only solution?

Many thanks!