In the US you have no expectation of privacy on work equipment unless otherwise stipulated. A hazy area is the use of work equipment at home but I would still error on the side of caution.
A long time ago when I managed our helpdesk I set up software that captured screen shots of employees. This could be set to every second to once a day. We did snapshots every 15 seconds most of the time. We could definitely read emails that you had up from personal accounts if they were on the screen. Also the software included a keystroke logger. We never used this at my work but it was available.
The screen shot software was used for two purposes. First to make sure employees were actually working during their shift. Not all of our employees had a supervisor on the clock with them. So if we got reports of an employee playing games all shift we might turn it on. The second is that an employee accusing another employee of bringing up improper things at work.
Note that we turned these things on. Some companies always have them on and some don't have the ability to turn them on. Purely company specific.
The more common issue is the use of router/proxy server at your work. I would not have the exact content but I would have the full use of the URLs you went to since all DNS requests would go through this. This is the most common thing that we look at with employees and is generally not that intrusive. We get IP logs for an address and see what DNS requests it made. Yes we see all sites but not what you were writing. Almost all employers have the ability to do this.
I know that our network team had to sign papers agreeing that if they found personal account information - really just passwords or account "secrets" - that they would not share or use that information in any way. So there was a degree of security behind it if you call it that. But really these were smart guys and they had your password if we turned on our spy programs.
Also note that about 5 years ago we started getting laptops equipped with spy software attached in the BIOS. So even if you reformat your laptop or do whatever with it we can find basic information from it if it is on the internet - it sends IP address, geolocation, logged on username, and last 100 DNS requests along with other stuff.
Should you check your Yahoo account at work? Sure. Should it have porn, nasty messages in it, other NSFW content or anything deragotory about your workplace. No way.
No one really cares about you looking at your email. It is really to what extent are you on email or facebook types of things. And also how harsh is the content. Will anyone say anything about your grandma's facebook posts about her latest recipes? No. Will people say something if they see your friends in swimsuits... Maybe.
You could just ask your boss and he will probably tell you the degree of monitoring. He could lie but most don't. I would expect that anything shown on your screen for longer than a second could be viewed by an employer. (And managers know that employees look at personal email and are OK with it. We also know that you open an email and then boom NSFW happens. Closing it right away is fine. This happening a lot is concern but otherwise its not a big deal. No one cares about your emails to your friends.)
