In todays market a forclosure can look like a bargain, and most are. The mold problem listed below is a nightmare waiting for an unsuspecting client due to one faulty valve. The remediation and repairs can easily top $50,000.00 in that case, so your question is valid. A few issues to note is that the previous owner could not afford the home so the defered maintenance probably wasn't a priority for some time. Bank's tend to close up a home and try to protect it from damage. The problem is that valves, hoses and flexable lines can crack when dry for long periods. The filters on the furnace haven't been cleaned in who knows how long and the firing systems on the water tank or the furnace can fail. Water tank's should be drained and refilled annually to remove the residue buildup, just let it cool and drain it to the yard with a garden hose before all that sludge fills your lines or fills your tank bottom.
More to your point is to run all the lines at the same time for a few minutes to check for adequate flow and drainage. In older areas roots can make their way into drain lines, eventually pluging the line or worse. Be sure the toilets flush at the same time, watch for leak's, even the little ones. Toilets should not move at all! Sewer gass is a bad thing.
Over time just do a periodic test monthly perhaps by using the fixtures, pee freely, it's your's!
It is estimated that the average home loses 300 to 400 cubic ft /min of conditioned or heated air hourly due to leeks in ductwork, if you have a dirty air filter that can double since the furnace/ac cannot get air easily from inside. That is the equivilent of 3 or 4 bath fans running constantly, sucking the money out of you wallet. CHANGE FILTERS MONTHLY!
