Depends on exchange rate: If fast/intermediate exchange then look for differential line-broadening in peptide spectrum when peptide is in excess. If slow exchange then might need to isotope label the peptide to see which resonances are shifted and/or broadened. In this context it helps to know the binding affinity by some orthoganol method. With a 'receptor' of only 11 kDa it might be possible to filter the spectrum to see bound peptide chemical shifts (assuming the 11kDa is isotope labelled).