#1
Hello, i have an old guitar without strings. is it possible to check the neck for adjuting truss rod, but without strings? how could i do this?
#3
Sorry, but you weren't really clear in what you are asking.

If you mean check if the guitar has a truss rod, then it is fairly simple.
If it is a steel string guitar (acoustic or electric) it probably has a truss rod. Look on the head of the guitar or inside the soundhole near the neck for a hex bolt to adjust.
If it is a classical guitar (flat fretboard, nylon strings), odds are there will be no truss rod. Check in the same way.

If you want to set the truss rod correctly, you need to put strings on it first, and if you are asking this question, you are probably better off taking it to a guitar tech since you can screw up a guitar pretty bad if you set the truss rod wrong.

If the guitar has a truss rod and no strings, and the neck isn't visibly warped, then the rod will need adjusting once strings are added.
^ Probably a load of crap

Gear

PRS SE Custom 24
Fender Squier Strat
Washburn D10SCE

Vox AC30C2
Roland MicroCube
Fender frontman 15G
#4
You usually need to have it stringed to set it otherwise there is no tension. You should probably string it and wait a while for the wood set in place before adjusting the truss rod as well. I've seen some strange devices that simulate the tension but I doubt that's worth investing in when strings are usually less than 10 bucks.
..I was watching my death.
#5
If no strings on guitar, just make sure the neck is "flat". Either sight it or use notched straightedge. If there is "back-bow", this means the neck is not straight and needs to be levelled. When levelled, and strings tuned to pitch, the string tension should bring the "up-bow" or relief to the neck. If not, the truss rod will bring that about. Good Luck!!!