Yellow watchman goby
 

How do I tell male from female??? I was at petco getting dog food and they had two in the same tank and they never leave each others side so I bought them hoping they are a pair... they now have residence under an old barnacle cluster in my tank

My pair are both yellow. I think the male is usually grey. If they hanging out together you probably got a pair. I've just got my first batch of babies through. It took a while as the larvae are tiney!

There is a great reefkeeping article on breeding them. Check it out.

If they were paired and not beating each other up, you've quite likely got a pair ;)