The dye test is of limited use, but go ahead and do it. If you see the dye on the surface, you will then be positive you have a problem. There could be things wrong that a dye test won't show. The best way of course is to dig up the septic and the leachfield, which would be unduly expensive and could destroy a perfectly good leachfield in the process. The owner would probably not allow that; I wouldn't. This is an expense you should bear yourself and that is the customary way it is handled. Most home inspections include a septic dye test. It would be harder for a buyer to trust someone paid by the seller to inspect anything, a bit of conflict of interest potential.