look for signs that show how serious a person is about you and your relationship and talk honestly about your expectations.
will the person run ar the first sight of obstacle? will s/he leave you when someone new comes along?
these are questions nobody can answer but you can attempt to seek out a person who takes your companionship as seriously as you do and always puts your needs first. It can be little things like making you soup when you are sick or picking you up from work so you can go on a date together..
Some people may not be able to put in that much effort but will show you in other ways that they care. They will want to know how your day went and build you up after a bad day for example.