Onan 8.0 89sp glow plugs-how can you tell if good?

[ED GARDELS]

I have to crank my gen set a long time before it starts firing and
then it sputters a while before it finnaly runs smooth. I just read a
post recentily that a glow plun can be bad and cause this problem. How
can you check them to see if they are bad? With a meter? Do you have
to pull them out?
Thanks in advance for any help-- Im still working on the ideas from
the forum on my telescoping , or non-telescoping steering wheel
Ed 89sp
Arlington Texas