The 21-year-old Leeds centre followed up his tryscoring debut against Wales with a hat-trick in Saturday's 44-6 win over France, which set England up nicely for next Sunday's final at the City of Salford Stadium.
Watkins looks sure to win his third cap in the rematch with France, but said: "Your spot is not safe. You never know what's going to happen. I guess it's going to be a real headache to pick a team for next week. I'm here just to work hard and improve what I do. I'm really happy to score three tries but next week is a different game."
With 124 points for and just 18 against in their two matches so far, England will go into the final as overwhelming favourites but coach Steve McNamara will not be getting carried away with their results so far.
England chose to play Wales and France after Australia and New Zealand opted out of the traditional end-of-season Four Nations Series in order to fully prepare for World Cup year and they have amply illustrated the gap between the big three and the rest.
England's bright young guns will face their true test in the opening game of the 2013 World Cup against Australia, but Watkins insists the French were no pushovers and is expecting an even tougher challenge next Sunday.
"It's still Test rugby at the end of the day," he said. "The game was physical despite the scoreline and we had to show some good attitude and determination at times.
"France really tested us defensively. In the second half we didn't help ourselves by giving penalties away but our defence was pretty solid and our attitude was spot on.
"They will be hurting from that so next week should be a good contest."