simple test on slope of lm()

Hello

I want to compare the slope (let's say 'b') of a linear model obtained 
with lm() to a theoretical value (let's say 'th').

To do so, I think I should compute a 't value' using 
something like :

(b - 'th')/standard.deviation(b)

and then look at the p-value of this computed t.

I don't understand how to do this in a simple way, just using lm() 
outputs for example. 

I'm sorry of such a naive/newbie question, and thank you so much for 
taking time to answer.

Fred
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