Cell not knowing Vegeta could wreck him is not enough to conclude he can't sense chi, as sometimes the warrior's chi sensing ability is reduced due to PIS. For instance, Perfect Cell also didn't seem to realize he would be screwed by ssj2 kid Gohan merely from sensing his chi, he had to feel on his own body that he was no match for the boy.
Alternatively, you could say that Semi Cell didn't seem to know he was outclassed from the get go due to Vegeta not have been exerting himself, and we know it's sometimes harder to get an accurate reading on the opponent's power when he is just there standing around and not exerting himself, as Vegeta and Nappa on Earth could serve as examples. #16 had a mechanical radar, and if we assume it works like a scouter, it doesn't seem to differentiate someone's standing around chi to their chi when fighting, hence why #16 knew Vegeta could be a respectable power.
