As long as you have enough data about the inlet conditions you don't need to assume whether the flow is laminar or turbulent (this is a very dangerous assumption), you can calculate Reynolds number to find that out:
$$ Re = \frac{\rho v D}{\mu} $$
where $\rho$ is the flow density, $v$ is the velocity, $D$ is the hydraulic diameter of the tube and $\mu$ is the dynamic viscosity.
Flow inside tubes is laminar for $Re < 2300$ and turbulent for $Re > 10,000$ (and transition in between), Assuming that you have a constant surface temperature tube $ T_s = \text{constant}$, making energy balance we have:
$$ Q^o = hA_s \theta_{lmtd} = m^o C_p \triangle T $$
Where $A_s$ is the surface area $\pi D L$ and $\theta_{lmtd}$ is logarithmic mean temperature difference.
For laminar flow, Nusselt number can be calculated as (for fully developed flow and moderate temperature difference): $$ Nu = \frac{hD}{k} = 3.66$$
For turbulent flow, you can use Gnielinski correlation, Dittus-Boelter equation or Sieder-Tate correlation. (This Wikipedia page is very helpful).
After calculating $h$ from Nusselt number correlations (You might need to assume an exit temperature for the first iteration) substituting in energy balance equation, after some iterations the solution will converge and you'll finally obtain the exit temperature from the tube.
(You can refer to the forced convection heat transfer chapter in this book for more details and solved examples.)
