Use 
select @@version
to find the version number

This will have a number which wil correspond to:

 6.00.121 v6.0
 6.00.124 v6.0 SP1 
 6.00.139 v6.0 SP2 
 6.00.151 v6.0 SP3 

 6.50.201 v6.5
 6.50.213 v6.5 SP1 
 6.50.240 v6.5 SP2 
 6.50.258 v6.5 SP3 
 6.50.281 v6.5 SP4 
 6.50.415 v6.5 SP5 
 6.50.416 v6.5 SP5a 

 7.00.623 v7.0
 7.00.699 v7.0 SP1
 7.00.842 v7.0 SP2
 7.00.961 v7.0 SP3 
 7.00.1063 v7.0 SP4 

 8.00.194 v2000
 8.00.384 v2000 SP1
 8.00.534 v2000 SP2
 8.00.760 v2000 SP3
 8.00.2039 v2000 SP4

 9.00.1399.06 v2005