ILC is always harder to detect, it's true. Mets to other regions, however, are NOT harder to detect, so you're fine with the usual testing there — which is basically blood tests, manual exams (to feel your liver, etc.); and reporting anything unusual to your doctor (bone pain, headaches, etc.)
As for a recurrence, if you had a mastectomy it's unlikely. However, you're more likely to have a new ILC in your other breast, than you would be if you had been diagnosed with IDC. AND you're more likely to survive it, than you would be if you had IDC—which is the balance. ILC, statistically, has a higher incidence of appearing in the other breast, but a better survival rate. And really, the only way to detect ILC in the breast is by regular exams from your doctor, and regular mammograms. You might also ask about alternating MRI with mammograms; although it produces a lot of troublesome false positives, MRI also pickes up masses that mammmgrams miss.
I'm an 8-year ILC survivor - hang in there! And good luck - PJH