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Fossil groups openParFGsclosePar present a puzzle to theories of structure formation. Despite the low number of bright galaxies comma their high velocity dispersions and high T_X seem to indicate cluster-like potential wells. Their measured c200 are high indicating early formation epochs comma in contradiction with the observed lack of expected large cool cores. We have proposed a discriminatory test comma using the intracluster light to mass ratio openParICLf-MclosePar in FGs comma using XMM and HST. The pilot study was successful and suggests that FGs are not merging and show high ICLf-M ratios in comparison to relaxed systems. We propose to increase the sample of bonafide FGs complementing XMM and HST archived observations to account for sample variance comma allowing us to reliably constrain their formation mechanisms and ages.