Yeah watch the tack, if the revs increase and you don't feel like your speed is increasing, or if you watch them go up right after you shift, only to come back down, or the revs stay put as you start to hook up.
If you have a stock clutch. . .you're slipping. . .guaranteed. If you're gentle, it will work, but if you get on it, that clutch and pressure plate aren't up to the task.