Both of these phrases can be used as a joke but also seriously. It depends on your tone of voice. If you laugh wile saying them then it let's the person know you are half-joking.
Asking if someone is cheating or if there's another person has the same meaning. Except you may say 'there's no one else right?' with a slightly more serious feeling.
どちらもジョークとしてもシリアスにも使えるフレーズです。
これは声のトーンによります。笑いながら言えば、冗談半分と伝わります。
"You're not cheating on me, are you?" も "There's no one else, right?" も同じ意味ですが、後者の方がほんの少しシリアスな感じになるかもしれません。
I hope you don't have eyes for anyone else.'
This playful term comes from the expression 'I only have eyes for you'. This means that the only person you see or are interested in is your partner. You can reverse this expression by saying ''I hope you don't have eyes for anyone else but me.'
'You better not have wondering eyes.'
wondering: can mean curious or express a desire to try something else.
Again, we are using an expression involving the word eyes to indicate that the only person your partner should be looking at or interested in is you.
I hope this helps :)
I hope you don't have eyes for anyone else.'
おどけた言い方ですが、この元は「I only have eyes for you（私に見える（私が気になる）のはあなた（パートナー）だけ）」という表現です。
「I hope you don't have eyes for anyone else but me（私以外の人に興味がないといいけど）」と反対に言うこともできます。
'You better not have wondering eyes.'
wondering:「curious（好奇心のある）」や、別のことに挑戦したい気持ちを表します
これも "eyes" を使った表現です。「あなた（パートナー）は私以外の人を見ては（興味を持っては）ダメ」という意味合いになります。
参考になるといいです。
All of these sentences would be suitable to ask in either a serious or joking way the only difference would be the tone of your voice and facial expression
You could also ask if they are seeing anyone else, this can also be asked in a serious or joking manner
To simply say "your not cheating on me are you?" would be enough for the other person to know what you mean
三例とも、真剣に聞く場合も、冗談として聞く場合も使えます。唯一の違いは声のトーンと表情でしょう。
「You're not seeing anyone else, are you?（私以外の人と付き合ってないよね）」と聞くこともできます。これも、真剣でも冗談でも使えます。
シンプルに「You're not cheating on me, are you?（浮気してないよね）」と言えば、十分あなたの言いたいことが伝わるでしょう。
Sometimes when a significant other is looking at their phone, we sometimes say, jokingly, "Are you talking to your other girlfriend"?
We also use the expression "wandering eyes". Wandering eyes means the other person is noticeably attracted to other people.
"Do you have wandering eyes?"
恋人が携帯を見ているときに、冗談でこう言ったりします：
"Are you talking to your other girlfriend"?
（他のガールフレンドと話してるの？）
"wandering eyes"（目をきょろきょろさせる）という言葉も使います。
"wandering eyes"は、相手が明らかに他の人に魅力を感じているときに使われます。
"Do you have wandering eyes?"
（他の人が気になるの？）
Native speakers will say like this: “Oh, you are cheating on me, aren't you?”.
There are lots of other ways to say it, but this one is the most accurate and most commonly used.
Here is an example on how to use this in a conversation:
A: Oh, you are cheating on me, aren't you?
B: Yes, I'm cheating on you haha
ネイティブスピーカーは以下のように言うでしょう：
“Oh, you are cheating on me, aren't you?”
（浮気してるでしょ？）
他にもいろいろな言い方がありますが、これが最も正確で、よく使われます。
会話例です：
A: Oh, you are cheating on me, aren't you?
（浮気してるでしょ？）
B: Yes, I'm cheating on you haha
（うん、してるよ。はは。）
I have see you flirting with other men and probably cheating on me, aren't you?.
I sincerely hope you are not cheating on me.
In the first statement, I have used the verb to 'flirt', which means to behave as if someone is sexually attracted to another person although in a joking way. However, if this happens in front of one's spouse or lover, it may send the wrong message. You may also put it in such a way that you don't sound like you are accusing him/her of cheating on you by adding "I sincerely hope you are not".
So, you may say:
I have see you flirting with other men and probably cheating on me, aren't you?.
or
I sincerely hope you are not cheating on me.
一つ目の例文で"flirt"という動詞が使われていますが、これは「いちゃつく」という意味で、冗談っぽくではありますが、まるで相手に興味があるかのように接することを言います。相手のパートナーの前でいちゃつくと誤解されるかもしれません。
浮気を疑っていると思われないように、”I sincerely hope you are not"（そうじゃないことを心から願うけど）と付け加えると良いでしょう。
”I have seen you flirt with other men, and (you're) probably cheating on me, aren't you?"
（他の男とイチャイチャしているのを見たことがある。浮気してるんじゃないだろうね？）
"I sincerely hope you are not cheating on me."
（浮気してないことを心から願うよ。）
You would let me know if you were seeing someone else, right?
When you want to half-jokingly ask your partner whether they are cheating on you, you can ask in the following ways:
-You are not cheating on me, are you?
-Are you seeing someone else?
-You would let me know if you were seeing someone else, right?
冗談半分でパートナーに浮気をしているか尋ねたいなら、次の表現が使えます：
【例文】
-You are not cheating on me, are you?
（あなた浮気してないわよね？）
-Are you seeing someone else?
（誰かほかの人と会ってないわよね？）
-You would let me know if you were seeing someone else, right?
（浮気してたら私に教えてくるわよね？）
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