This.See @Wasnex reply above.
It's a crap shoot...
IEM mixes can change from when the band does a sound check to when they start playing.. adrenaline and all that !
Ask the FOH guy HOW he wants you to communicate what you need once you start playing.
During soundcheck it may just be talking into someone's mic (if you don't sing) to tell them what you need.
Still, being handed a pack works great. If they have those, then hopefully they also have a means to let you control your mix, or give you access to do so.
However, it often [usually?] doesn't work well, and if you plan to do this on a regular basis, be prepared to be frustrated.
Which takes me back to this being the reason bands have their own system.
Simply showing up and attempting to implement an IEM system on the fly is a sure way to guarantee that it will fail and everyone comes away hating IEMs.
I'm often asked, we'd like to start using IEMs, how do we go about it? My answer has become, unless you are willing to invest in good IEMs, (not 40$ buds, in spite of some that swear by those things), good RF, and 2-3 rehearsals to set them up and make adjustments there, just don't bother wasting your time and money. NAd if you show on a gig and are handed a belt pack and generic earbuds, you are being done a grave injustice and you'll hate that very quickly.
The amount of close-mindedness with IEMs is overwhelming. I've grown to simply keeping my mouth shut when the subject of IEMs comes up. I love mine and have a few ways to implement them in various situations. And that's all that matters.