Check All time
Yes there are checks every where !! SO to JFO requires GND school sim and line training then a line check to pass and become a full FO (at full FO wage) . It is more dangerous than command as you will be asked to leave if you do not make the grade. You can fail the line check up to four times. Although this varies as some have been let go before then. This does not happen very often (as compared to Command failures) but when JFOs say their job is on the line it really is.
Howwever when you take command course you can be Cat D'd and stay on as an FO. Although that is subjective as if they do not like you, you will also be asked to leave.
There is also a line check and course to become a relief FO. As a regular FO you can be by yourself in the cockpit. As a relief you can be "in charge" while the Captain takes his rest (long haul)
You can also have said promotions taken away, as a result of a bad line check or in extreme cases a bad sim or line flight. Even heard of guys being demoted after crewing up (being called out to act as PNF for someone else's sim) !! Then you get a "heavy check" or management check ride and if you make the cut you can go back into the shadows. If not you are in the spotlight and eventually kicking your lunch box down the road. So don't screw up !
We have a seniority list but currently you can upgrade on the classic immediately (Direct entry) on the 400 Freighter within a few years and on the pax fleet after 9 or so years. The management does have the right to hold you back without any particular reason (could be political or training history related)
So there you have it.