1. Who do you like more - them, or you?
There really is no right answer to this question. If you love them more than you love yourself - it's an unhealthy relationship/will more than likely be an unhealthy relationship. If you love yourself more than you love them - probably just isn't the right relationship. I think a good relationship involves realizing that the more you love them, the more they love you - and the more you find yourself loveable. Does that make sense? Love is a roundabout thing - you shouldn't feel like you have to sacrifice the love you have for yourself OR the love you have for the other person. It should just be.

2. How do you prioritize your time? Alone AND together?
If you're rushing through your own personal duties to be with the person you love that usually isn't a giant red flag for an unhealthy relationship - it just means you're into them! But if you find that other areas of your life are lacking because you're making this person the center of your life - that is unhealthy.
Same can be said of time spent together. If you're interested in other things while you are together rather than just being together - it's probably headed down an unhealthy path faster than you think.
Time spent alone should be time to grow as an individual, time spent together should be spent growing as a pair.

3. How long have you been together?
You can more than likely gauge pretty quickly and early on in a relationship whether or not it's an unhealthy one. USUALLY, you see all the red flags - ignore them - then end up in said unhealthy relationship for WAY longer than you should. If it's the beginning stages and you're weary of the direction the relationship is going - you're probably not wrong to feel that way: trust your gut. If you're DEEP into a relationship and asking yourself if it's unhealthy/when it started to get unhealthy, you can probably pin point where things started getting iffy probably pretty early on. Be mindful of your heart and your mind while starting a relationship/continuing one as well. You know more than you think.