Funny you should ask that, I'm in a similar position with my PPL, though I don't have the ATPL (yet).
Here's the response I received from one flying school:
If it has been between 5 and 10 years since your SEP rating expired you will require the following training for revalidation:
Dual flying training with a flying instructor (P/UT), which must include:
1. Discretional instruction in instrument flying.
2. Discretional instruction in pilot navigation.
3. Discretional instruction in the use of radio navigation aids.
4. One hour stall/spin awareness and avoidance training.
Pilot-in-Command (PIC) flying which must include:
1. One hour solo general handling exercises.
2. Four hours solo cross-country flying which must include one flight of 150 miles with two away landings.
Theoretical Knowledge to include exams in:
1. Aviation Law & Operational Procedures.
2. Human Performance & Limitations.
Finally you would need to pass a Skill Test.
I'd assume, though you'd need to ask, that your ATPL would cover the exams, and then you'd need to do a couple of hours plus skills test
A different school, basically said spend some time flying with an instructor: general handling, stalls etc. an oral exam on the aircraft and a skills test.
Hope that helps.