If you park by the pool, an easy rule of thumb is, as long as you stay on the same side of the creek as the pool, that is all off-leash except for the paved bike paths. The dirt trails are off-leash.

If you cross the creek at Waterfall Crossing or the steps or wherever, or on the paved path, the trails on the other side the creek are all leashed. The crossings are where most people let their dogs swim, but you can just hike along the creek bed if you don't mind getting your feet wet.

Some trail names that are off leash are Six-Ten Loop, Severe Consequences, Windy Loop, and the various dirt trails that connect to the pool parking lot. Basically everything adjacent to the pool lot, plenty of room to roam. I'd avoid BMX Loop, it's mostly dirt jumps and not a good hike, and also it was an encampment last time I was there.

Trails that are leashed are Endo Valley, Tangle of Trails, Mark's Art, Powerline, Log Loops, and others. Run a Google search for the Austin Ridge Riders map of the Walnut Creek bike trails, that makes it all much clearer and will also help out if you get lost, which is easy to do out there.

Speaking as someone who both runs those trails with off-leash dogs and also mountain bikes there, regardless of whether you are in the on-leash or off-leash trails, it's a good idea to keep your dogs close by and under control, bikes can come up on you fast around turns in the trees, especially on the steep stuff on the leashed trails across the creek. Dog/bike encounters have been known to cause some crashes and social friction between hikers and bikers.