var result = promotionEngine.Evaluate(entryReference, market, currency, RequestFulfillmentStatus.Fulfilled | RequestFulfillmentStatus.PartiallyFulfilled);

Should tell you all promotions that are applicable to a specific entry, filter those to find the gift ones. Be warned that it isn't cheap to run that for every line in your cart.

A better solution would be to add partially fulfilled rewards to your actual call when you apply the promotions to your cart:

var result = promotionEngine.Run(cart, new PromotionEngineSettings { RequestedStatuses = RequestFulfillmentStatus.Fulfilled | RequestFulfillmentStatus.PartiallyFulfilled});