it may be as simple as not drinking enough fluids, that can cause your urine to go dark and strong, also he might have an infection... I have just left me partner who was addictted to heroin and his urine was strong at times and other times it was normal... What i found was with him he lost his desire to eat meals or if he did eat he could not finish it. He also craved sweet things like sweeties and chocolate.. The weight fell off him quite qucikly and his behaviour changed, he would go out at different times, for an hour or so, he started spending money like water and had nothing to show for it, he became quite irratable and grumpy and ill when he was needing more.
you can be opaite tests on-line, but dont do it behind his back, speak to him and tell him your fears, trying to catch someone out is a visous circle as i well know, you end up driving yourself bloody crazy... goodluck and i hope your partner is honest with you xx