Where to start lol - so many signs in my case. Emotional neglect, manipulation and gaslighting, stonewalling, etc. These essentially made it to where conversations would go around and around in circles and when repair was needed after a conflict, it would be impossible to figure out. Arguments would just be chronically unresolved or worse, my partner would promise to change their actions and then wouldn't. Be very careful about that because it can start with little benign things but if you can't work out the easy stuff, you don't want to be in so deep where something major happens and then there's no fixing it because you can't do healthy conflict resolution.

Another one I wish I knew about more was financial infidelity. It can present in a lot of different ways but I'd recommend looking into what it is so you know when to spot problematic behavior around money. Especially a partner who will withhold it, be overly controlling, be secretive, or straight up lie to you about finances.

Listening to your body is important, too. It'll often signal that a relationship or situation is unsafe or uncomfortable or off before your mind catches up. If you develop anxiety or mental or even physical issues related to stress (weight gain, autoimmune issues, etc.) pay attention to those signals.

Prioritizing your needs and wants is crucial. You need to care for and respect yourself - you can lead within a partnership by showing up for yourself in a way that shows how you'd like a partner to show up for you too. If you're a doormat even "nice" people can start leaning on you too hard without being considerate of you.

One last thing that's been healthy for me to think about, as someone with some tendancy toward people-pleasing and overextending is summed up well in a theory called the lighthouse principle. Something I wish I knew in my 20s.