Just guestimate it?
It really depends on many things. If you have a certain concept which will work with that combination of classes, AND you give it WBL of at least a 10th level player, then of course you're at CR10+.
If you give it Elite Array or a point-buy you're probably at CR12 or more.
If it ends up with a lot of useless abilities (For instance Int-based casting with 10Int) and doesn't get WBL, then probably you can easily ignore the Beguiler levels mostly, because all the PCs need to do is hit it a little more. So halving and rounding down the levels for the CR calculation seems fine. So unless Assassin levels DO fit the concept, you could also cut those in half, I guess, or use a two-thirds progression, perhaps.
My suggestion:
Non-concept, actually unassociated, even above HD=Class levels: Everything halved, CR 9
Conceptually sound but levels don't match up well with the critter: CR10
Conceptually sound and stringent, but no WBL and no good array: CR11+ (Like Beguiler 10 with at least 15-16Int with items)
Everything fitting into place and WBL for class levels: CR14 or even higher