How find out if bathroom sink drains in gray or black tank

My bathroom sink was plumbed into the black tank. Shower was plumbed into grey tank. Mfg said I had the only of that line plumbed like that and it was due to the way the pipes were run. Can easily check by opening grey tank (sewer hose attached) empty what is in there, then turn on bathroom faucet. Running out? Yep, grey tank. Nothing coming out? Black tank. Can go one step farther and do the black the same way?