Lex watches Clark and bides his time. Clark keeps lying, but Lex never calls him on it anymore. It wouldn't do him any good. Clark would find a way to bumble around with a lame explanation. Then Lex would push and Clark would get mad, and Lex still wouldn't get the answers he wants. So, Lex doesn't push anymore.
Despite the lies, Lex still loves Clark more than he loves any other person alive. The depth of his feelings for Clark sometimes scares him and sometimes it humiliates him. Sometimes, it makes him feel like he's on top of the world.
If Clark wants to lie, if that's all Lex's friendship means to him, then so be it. Lex will keep observing and gathering information until he has something definitive. Something that he can show to Clark and say, "Now, you can't deny me anymore. I know the truth and this is it."
That will be the last chance he will offer Clark to tell the truth. When that day comes, it will either be Lex's most glorious day or his most devastating one, depending on Clark's reaction. If Clark comes clean, Lex will forgive him for all the lies and the deceptions over the years. He'll take Clark in his arms and tell him that together they will truly be the stuff of legends. He knows the way Clark looks at him, but Lex won't take him to bed with deceit between them.
But if, on that day, Clark so much as hesitates about admitting the truth; the truth that Lex already knows and has proof of in his hands - then Lex will banish Clark from his life and from his heart. He'll rebuild all the walls that Clark knocked down. He'll become the man his father always wanted him to be and the person Clark is terrified he'll turn into.
And it will be Clark's fault.
Until that day, Lex will watch and wait and gather.