Why couldn't Lyta just... [possible spoiler]

She was able to scan Kosh, whose P-level is off the charts. And nobody knew Sinclair's mind had been altered, let alone by a P-12 telepath, so there is no reason why he wouldn't insist on being scanned. And unless he had been programmed with false memories of attacking Kosh (which he would have remembered), the outcome of the scan would at any rate work in his favour. If he consciously remembered being stuck in an elevator -- which he did indeed remember -- then this would show up in a scan, no matter how altered his mind might otherwise be. If there was some sort of telepathic block preventing a scan, a stronger telepath might be summoned (that block would alone be sufficient to warrant further telepathic examination).