In my experience, it varies according to the venue and the specific event and the "participants".
It always helps to read up on specific guidelines. Otherwise, you can try to get in some nice gear with longer lenses and be turned back. You may be able to check it in (and pick it up afterwards) or you may have to walk it out to your vehicle, which in "pro" venues can be quite a walk!
'Course, there are different rules that apply to "pro" photographers, those licensed by a media outlet and specifically allowed to come in, take a position with high-level gear and shoot away!
In practice I've found that things vary. I read the guidelines, then I'll "play it by ear". Sometimes you can get away with things a bit, if you are willing to take the risk.
For example, I sat in the bleachers for a pro basketball game and had only and old compact camera to shoot with. Understandably, the results were not so good!
Then, I was headed to a AAA semi-pro baseball game, and I checked and double-checked with a phone call, and my gear was allowed, including my "workhorse" 1DM3, the 70-200 f/2.8 white L lens, and along with them my longer 100-400 white lens, and thanks to the openness of the venue, I was quite happy with my results, although I would have been happier if I had been up alongside the TV and pro crews that were perched along the sideline fences!
I took a trip to Disneyland a number of years ago, I brought a pack/bag of "modest" gear, along with my compact super-zoom camera. They let me in, although I pretty much relied on my compact for shooting on the go there!
And then, there are indoor performance events where there are both limits on the size of your gear and also no-flash rules.
In smaller venues, such as some smaller sports events and also the above indoor performance events, it helps to get to know the folks that are "running" things! Indoor performances, for example, may have special accommodations for "working" photographers, but if you are in the crowd waving around a camera, especially a bigger one or using a flash, well, you can get escorted out!