Our main bit of advise for you - estimate!
Admittedly it won’t be perfect but it's much better than just considering it a write off and eating as much as you'd like.
Also, make an informed decision with the restaurant that you pick, perhaps look up the menu online in advance to check that they have something appropriate.
Try rewatching this video 'Eating healthy on the go' as it contains pointers on what to aim for when choosing food at a restaurant and may be useful in this situation.
You can get a useful tool from carbsandcals.com which has a free downloadable pdf on 'eyeballing it'.