You can check that in the phone app too. Hit new message, enter the numer, hit "New message to… " and it’ll tell you if it isn’t known. There is rate limiting in that function, you’d need a lot of signal accounts to sweep all phone numbers.
You could also try signing up to signal using the number you want to check.
Neither way however you would get the signal name or profile pic of the number if I understand it correctly, that would get sent if they reply to you.