I worked in a UK studio 40 years ago that had both, 12" and larger hot lamp fresnel units and a number of them that had been converted to electronic flash. The easiest way to tell was look at the exposure settings written on the 4x5 negative sleeves. In winter the hot lamps were handy to have. Other than the size of peoples pupils, it was impossible to tell, the lighting effects were identical, as they should be considering the light units were identical except for the bulb holders.