If your Steem power is below 500, you can't do so from steemit.
Steemit provides vote percentage slider to those who own more than 500 steem power.
Steemit doesn't provide this feature as they think account upvote which has less than 500 sp is nothing more. If you have less than 500 and you vote someone, that will charge 1005
But if you really want to determine you vote strength, than you can use Busy.
Busy is working with steemit.inc and Busy has better Ui preference and many more features. Just go to https://busy.org and login with you steemit master or private key.Then go on setting or just click on this link- https://busy.org/settings .
you will see a option of enable/disable slider option. Enable it. Now whenever you try to upvote post busy will ask confirmation of voting strength.
With out busy there are many more front end and dapps from which you can determine your vote strength.