Watch to see if Cosmo keeps the third right (clockwise) arm curled and does not use it for trying to locate food or moving around. The tip of the arm is modified and will have a slightly different look and there will be a channel along the arm but both are very hard to detect in a living animal. If this arm (hectocotylus) stays curled and is not used in day-to-day wanderings then you have a sexually mature male. If you don't see this behavior then Cosmo is either a female or is not yet sexually mature.
You might take a walk through Kalypso's journal (the link is to the page where the arm was severed, you will have to look to see if he posts when it recovered). He lost part of an arm in the tank and I think AM mentions when the arm had recovered. If you can dig up the thread on Hex, (vulgaris) he came with only 6 arms and fully regrew them. I can tell you that is it not overnight and the first thing you will likely see looks like about an inch of string at the end of the arm. The arms grow length before width and there is often a misaligned or missing sucker where the truncation originated.
Thank you for the info dwhatley. I read through the journals of hex that I could find it looks like he got it with missing legs on Sep 12th, 2006 and he said that they were fully grown back on Nov 22nd, 2006 that’s days 72. This is probably something that varies for different Octopuses.