Probably the most important thing to check is their audience and reach.
Also, start a conversation and see if they respond to your liking. They should speak your language too.
The last thing would be to explain them how it will benefit them and you both. In an ideal situation you both gain something from it, like more subs/clicks/new fans/cliënts.
They do not have to be the same "size" in the online world as you, but it could help to have about the same amount of followers and "grow" together.
Also, the niche does not have to be the same per sé, as long as the audience overlaps.
Hope this helps ;-)