They test for levels not synthetic replacement. If the HRT used replaces only to minimal levels (i.e. Not even those of a healthy 40 yr old woman) then testing will reveal nothing. The testosterone replacement therapy I am on (under a physicians care) does not bring my T levels anywhere near levels considered suspiciously high. Females aromatize testosterone to estrogen so by simply replacing testosterone via subcutaneous pellets, both estrogen and testosterone levels are high enough that I no longer feel worn out and continually fatigued.