The phrase "firstly check" is grammatically correct and can be used in written English.
It is commonly used to introduce the first item or step in a list of things to be done or considered. Here is an example: "Before submitting your application, firstly check that all the required documents are included."
Firstly, check with the Office of the Deputy Prime Minister to find out if you need planning permission.
If you're already feeling fatigued with this year's gift stash then firstly, check your privilege, then settle down to listen to some hot new tracks!
Firstly check when and where to see the species you are interested in and try to pick some reasonably warm weather.
The filter will firstly check whether the first point is noise.
Firstly, check the stationarity of the panel dataset by using Levin Lin Chu (LLC) and Im Pesaran Shin (IPS) unit root tests.
When receiving such an Interest, the en-route router will firstly check whether the requested content is cached locally, by looking up CS.
Receiving the message {ID u, E k (PS′), yP, b, T3} at the time T′, GW firstly checks whether (T′-T3) ≤ ∆T holds.
The applicability of the MOX membrane sensors for determination of the drug in the dosage forms was firstly checked by the studying the recovery of an accurate amount of pure MOX in solutions.
An Interest packet that arrives on MaFIB is firstly checked in the MBF group in parallel with the LPM to determine whether the forwarding information of this Interest packet is in MaFIB.
The applicability of the MOX-membrane sensors to the determination of the drug in the dosage forms was firstly checked by studying the recovery of an accurate amount of pure MOX in a reconstituted powder samples.
Regarding the equilibrium merger, it is firstly checked that a (H) merger is always leading to larger profits for participants, however, a (V) merger does not in any case.
