weigh up

Never Let Me Goの主人公、KathyにRuthが馬に乗らないかと言って来ました。
'Do you want to ride my horse?'
I was in the midst of playing with two or three others at that point, but it was clear Ruth was addressing only me. This absolutely delighted me, but I made a show of weighing her up before giving a reply.
ここに出てきた "weighing her up" は初めて見る表現です。　この表現はこの場面の他にも次ぎの様に使われていました。
When she did speak, Miss Lucy seemed to be weighing up each word carefully.
"weigh up" は文脈からすると「吟味する」、あるいは「評価する」と言う様な意味の様ですが、辞書で確認します。
・Collins English Dictionary: to make an assessment of (a person, situation, etc); judge: Borrowers need to weigh up the immediate benefits against the cost associated with the mortgage.
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: to find out the weight of something. I can't tell you how much this will cost until I weigh it up. Liz weighed up the meat and jotted down the price.
・Cambridge Dictionaries Online: to form an opinion about a situation or someone's abilities and character by thinking about them carefully: I weighed up my chances of escape, and decided to wait for a better moment.
・Oxford English Dictionary: weigh someone/thing up: carefully assess someone or something; the coach weighed up his team’s opponents
考えて見れば、重さを計る事は評価する事に通じるのは当然ですね。もちろん、重ければ良いとは限らず、軽い方が良い場合もありますね。