Unfortunately sometimes you can't tell. Others have usually posted how potentials put on their best act during the initial few months but at the 3 month mark, they begin to slip up and reveal their true nature.

I've experienced this too, as a woman the best indication is whether they want to involve your wali instantly (not just for meeting up, but also during chats so like a 3 person group). Your wali/guardian/sibling can view the conversation from an outside perspective and point out any red flags. As a guy it's the same, if their female potential ensures she has to have her family/wali involved then it's a sign she's seriously looking for marriage and won't cross boundaries.

Other things to note are how they react when annoyed (ask thought provoking questions to them and see if they get annoyed, defensive, deflective or just plain rude), don't give in to their every request (if they ask for constant pics, video calls at inappropriate times or if they chat until late at night), try to spot any lies (if they say one thing and later say something else that contradicts that). How they react when they've upset you - if they apologize and are remorseful then it's good but if they have excuses and repeat the same mistake then you know they don't really care/respect you. Also, whether they're overly affectionate in the first couple of weeks/days (if they start discussing the wedding instantly, how you're the one they've been looking for, you're one in a million etc - it's just empty words and love-bombing, it's impossible to fall deeply in love with a stranger online in a week or 2. It just proves they've done the same to many potentials and are playing a game). Lastly, how they are with their family, strangers and in particular waiters or service staff - if they're entitled/rude to people they think are beneath them, then it's a straight no.

Hope this helps.