Anelka fired his first two Albion goals in last night's 3-0 triumph over Puskas FC Academy.
"I never had a doubt about Nicolas because I'd worked with him at Chelsea," said Clarke to the WBA website. "I was with Avram Grant when we bought him in the January.
"So I knew the type of person he was, the type of player he is. Since he clearly looks after himself I had no doubts. I've one or two good contacts around the football world and I checked him out when he was at Juventus.
"The fitness statistics and information I got about his fitness was that he was one of the best trainers.
"He was one of the top performers, in terms of fitness stats, at Juventus.
"If you can do that at Juventus then you're fit enough for us."