That´s why this thread was created. In the main THS platform, we as sitters cannot send messages to other sitters, as far as I´m aware. But in the forum we are able to DM each other. Since you have your sitting profile embedded in your forum profile, people can take a look at your profile and get in touch with you if they think you may be a good fit with them, and visa versa. Then, if you´re both interested, you could arrange some online calls and at some point meet up in person to see if you could feel comfortable and confident enough to give it a go together. Or you could try doing a short sit together somewhere mid way between the two of you, if you live pretty far away from each other and would be up for that experience.
If the other sitter has a number of great references from previous sits and the sits look like the kind of ones you might pick yourself, that would seem to be a good indicator as to how reliable and fitting they may be. Along with the rest of their profile and your one to one communication exchanges.