How to do I check if my tubes have slime in?

[Acajou]

I want to put slime in my new (to me) DR's tubes, but I'm not sure if there may be slime in already. I don't want to overfill the tube with slime and mess with the wheel balancing. Is there a way to check if the tubes have slime in without removing the tube? Should I just put some new slime in anyway? I'm guessing any slime that's in there will be older than 6 months

 

[Gerard]

 

[TVB]

 

[Acajou]

 

[TVB]

 

[Acajou]

 

[TVB]

 

[Acajou]

 

[2wdrift]