This is discernible due to sudden albeit momentary change of facial reaction that is often masked by bill payer's efforts to divert attention by loud speech, reference to other events, reassurance that bill amount is far less than expected,etc.
This reaction may be excessive due to unexpected amount or ever so slight even if the said amount was presumed. This reaction is only involuntary and genuine on the face of actual payer at the time of bill payment and will occur with certainty upon bill payer's face 97% of the time regardless of financial status.
One of Mark's friends Dave did mention to another mutual friend Lisa:
"Hey great evening we should do this more often, next time it will be my treat (secretly thinking heck no) but I'm glad Mark is treating us today ..here comes the check..watch for it ..watch for it... BILLFACE!" as he & Lisa quietly snickered in the corner.
Mark quickly paid the bill and began to mention how he will have so many stories to tell his co-workers tomorrow (trying hard to disguise his bill-face effect).