Bathroom was remodeled, toilet and sink were moved, when sink was connected there was some water going into the basement - now I don't use sink. Contractor drove me crazy, he is unreliable says he'll fix tomorrow doesn't show up. Now I would like to know possible source of leak before I bring in a (new) plumber. Thanks
There are a few possibilities but you have to eliminate them one by one. The first is to determine if this is a "supply side" leak or a "waste side" leak.
If it is supply side, it will leak regardless of whether you are using the sink or not. It will likely be a lot of water and may be accompanied by hissing or sizzling noises beneath the sink, near the copper lines supplying the pressurized water.
If it is a waste side leak it will not occur continuously, except in a few conditions, and should not be a lot of water. It will most likely puddle directly beneath the sink, on the floor or the tile. It will be worse when you use the sink and potentially a lot worse if you fill the sink to the brim and then pull the stopper. If the bad connection is further down the waste line, it will puddle in the basement or ceiling of the basement.
Pics of the sink from above and below will help. Is this a pedestal sink or vanity?