Trees, Shrubs and Conifers: Meyer Lemon not fruiting - is it rootstock that's growing?, 1 by paani

paani wrote:
This is a 10 yr. old Meyer Lemon, new to our garden.
It flowered but never fruited for original owners.
Have only thinned it a bit thus far, for light.
Healthy dark green foliage. About 8 ft. tall. And it's got the hypodermic size thorns.

How can I tell if it's rootstock that's growing?
(often non-fruiting mandarin, I read somewhere. true?).

There are four trunks. One looks like it could be grafted, but it's up very high.
No visible graft on the others. (mind you, I don't know anything about grafts).

Could it be a mix -- one grafted Meyer lemon trunk plus three interlopers from the rootstock?
They said it was "just a stick" when they planted it, so it must have been single trunk.

Should I check for a graft under soil level?
Or is own-root possible? (not common for Meyer Lemons I read...)

Any advice on how to diagnose this - and get it to fruit! -- much appreciated.

pics. are: 1) whole tree, 2) the 4 trunks at soil level, 3) the possible graft