I was outside examining mine yesterday and tried to take a pic but the batteries dies - oh well. My lilacs are just starting to form flower buds now and they look like a teeny tiny bunch of dark purple grapes with normal leaves wrapped around them - so I would say that there is probably no way to tell before the little purple (if your lilac is purple - ha) flower buds come out. That I can see anyway.

I was examinging buds yesterday, as well, and could see no difference. My lilacs are transplanted from seedlings about a foot tall. They are in their third year and about 4 feet high. Anybody have any idea how long I will have to wait for them to flower or if I can do anything to speed the process? Thanks.

Make sure they are in full sun. I've seen them fower at only 2'-3' tall.

i would like to no the diff. also.
i have a real nice lilac bush in my landscaping.
I took some soft wood cuttings from it and they are doing really well. I hope to have blooms in 3 yeas or so.
Sofar i have not known any thing to speed the process.