It depends on who your LoR writers are. I'm assuming they're former professors of yours with whom you've taken one or more courses with where you've illustrated your capacity for carrying out work on the graduate level?
Personally, my letter writers were professors who worked in the department I majored in, and I had worked in the departmental office while in undergrad, so they knew me quite well. I'd left after graduation to teach English abroad for a year while I was defining my interests in grad work, so I just popped by the department to say hello and tell them that I was applying and if they'd consider writing a favorable rec. If I had instead contacted them via email or phone and wanted to follow-up, I might just pay them a visit and pursue a little small talk and then pop in some questions that I'd encountered while in the process of applying. It's well into the admissions season now (at least in America), so you've likely been writing (if not, preparing to write) your statements of purpose, so you could sincerely ask for their input on helping to illustrate your interests in the statements, or ask them pertinent questions about persons of interest at the programs you're applying to, funding issues, etc.
I certainly wouldn't suggest that you feign interest just to allow yourself to foil out the strength of your candidacy for grad work, but if you do have pertinent questions, it might be good to just ask to meet with them (or pop in, depending on how well you know them) to discuss them. Good luck!